1. When a bus stops suddenly, passengers can fall forward due to inertia. Their feet stop moving with the bus but their upper body continues forward momentum until impacted by another object. 2. How far a kicked football travels depends on its air pressure. Higher pressure balls maintain their shape on impact and bounce farther, while lower pressure balls deform on impact, losing kinetic energy as heat. 3. It is difficult to balance a stationary bicycle due to its narrow base. However, moving introduces rotational momentum, allowing the rider to counteract tilting through slight handlebar adjustments and maintain balance.

1. Physics at Work and Play 1
Physics at Work and Play
1. A matter of inertia
Many of you commute to school by bus. Sometimes, when all the seats are full
you have no alternative but to keep standing, and in a moving bus you think
it's safe to hold on to the overhead rod. Otherwise it's quite difficult to keep
your balance. You're quite right. By holding on to the rod you can save
yourself from toppling over when the bus turns and stops. But if the driver
suddenly slams the brakes, the sudden jerk could make you lose your
handhold and fall forward over your friends, some of whom may have
tumbled to the floor as a result of the jerk.
Why do you fall forward when a running bus stops suddenly? The simple
answer is: Newton's first law of motion, which states that "an object at rest or
travelling in uniform motion will remain at rest or travelling in uniform
motion unless acted upon by a net force." This law is also called the law of
inertia. When the bus moves, both the bus and its passengers including you
also move together with the same velocity in the same direction. When the
driver applies the brakes smoothly, the bus slows down along with all its
passengers, ultimately coming to a stop. You don't feel any sudden jerk.
But when the driver applies the brakes suddenly, the bus along with your
feet, which are in contact with the floor, come to rest instantly. But the inertia
of the upper part of your body keeps it moving forward and as a result you
fall. The same principle applies when you get down from a running bus.
Unless you run forward a few steps to slow down your motion you'd fall
forward because your feet comes to rest instantly on touching the ground, but
the rest of your body keeps moving forward.
2. Air pressure of football
Football is a game most of us must have played in our childhood. It is a
common observation that if football bladder is not adequately filled with air
the ball does not go far even when kicked hard. A ball pumped stiff to its full
capacity goes much farther even with a moderate kick. Why does the distance
a football goes when kicked depend on the air pressure?
The ball used in football games is an air-filled sphere with a circumference of
68–70 cm, a weight of 410–450 g, inflated to a pressure of 60–110 kPa (or 8.5–
15.6 psi). The design of footballs has changed over the years. The ball used in
the 2006 World Cup was of a new 14-panel design that replaced the
traditional 32-panel hand-stitched balls. The new design has fewer seams, so
the ball is rounder and performs more uniformly, regardless of where it is hit.
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2. Physics at Work and Play 2
The more perfect the sphere is, the more balanced it will be, offering greater
accuracy due to the predictability of its flight. But more than the shape it is the
air pressure inside that is important.
How far a football goes when kicked depends on its bounce, which in turn
depends on the ball and the surface it hits. Balls with air inside, such as
footballs, bounce very poorly if they are not pumped up – no matter how
hard the surface they hit. This is because a low-pressure ball gets deformed a
lot when kicked with the boot sinking into the leather surface. This wastes a
lot of energy, which is converted into heat and lost to the bounce. A properly
inflated ball doesn't deform much when hit, so little energy is lost bending its
skin and the bounce is more.
With a pumped-up ball, the kinetic energy of the boot is changed to potential
energy stored in the air molecules inside the ball at the instant the boot hits it.
Quick as a bounce, the potential energy is released on rebound and turns into
the kinetic energy of a rebounding ball, which goes much farther than a
poorly inflated ball.
3. Balancing a bicycle
Bicycle, or the bike, is a common mode of transport in cities and villages alike.
In both biological and mechanical terms, the bike is extraordinarily efficient.
In terms of the amount of energy a person must expend to travel a given
distance, it is the most efficient self-powered means of transportation. From a
mechanical viewpoint, up to 99% of the energy delivered by the rider into the
pedals is transmitted to the wheels. But you can't balance on a bike standing
still; if you try you'd fall down. To keep the bike balanced you have to keep it
moving. Why is it so?
Different principles of physics are involved here. If you were on a stationary
bike it'd be stable as long as the vertical line from its centre of gravity falls
within its base. But in this case the base is extremely narrow – only a few
centimetres wide. So even a slight tilt would bring the line dropped from the
centre of gravity out of the base, making the bike unstable. Now, suppose you
find the standing bike leaning to the left; your natural tendency would be to
lean to the right to counterbalance the lean. But in moving the top of your
body to the right, you'd be actually pushing the bike to lean more to the left,
according to Newton's 3rd law. So it'd be almost impossible to stop the
leaning bike from falling when it is standing still.
On a moving bike, however, rotational momentum makes the bike easier to
balance. By slightly turning the handlebars right or left, you impart some of
the rotational momentum of the front wheel to rotate the bike around its long
axis, the direction in which it moves. That way you can counteract any
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3. Physics at Work and Play 3
tendency of the bike to topple to one side or the other and keep it balanced.
The faster it moves, the harder it is to make the body of the bike change
direction and you have much more time to make slight adjustments in body
position to prevent the bike from falling over. That's why a moving bike is
stable.
4. The clinical thermometer
Whenever anyone has a fever a clinical thermometer is used to measure the
body temperature. But before the thermometer is used why is it necessary to
shake it? Well it's to bring down the column of mercury and make it rejoin
the mercury in the bulb and reset it for making a new measurement
Like any other mercury thermometer a clinical thermometer also has a
mercury-filled glass bulb and a graduated glass capillary into which the
mercury rises to show the temperature. But in a clinical thermometer there is
constriction at the point where the capillary joins the glass bulb. This
constriction is meant to cut off the mercury column from the bulb when the
mercury in the bulb shrinks.
When the bulb of the thermometer is kept in contact with the skin under the
armpit (or under the tongue in the mouth), the mercury in the bulb expands
due to heating by body heat. The expanding mercury rises in the capillary.
The height up to which the mercury column rises depends on the temperature
of the body. After the thermometer is removed from the armpit it is
immediately exposed to room temperature, which may be lower than the
body temperature. If there were no constriction in the capillary the mercury
column would start falling immediately and would not show the actual
temperature of the body. But the constriction, which is narrower than the
capillary, breaks off the mercury in the bulb from the mercury column in the
capillary. This happens because, during expansion, there is enough force to
push the mercury up through the constriction. However, during contraction,
the forces pushing the mercury back down through the capillary are too weak
to force it through the constriction. As a result the mercury column breaks.
Since the top portion of the mercury column is left almost undisturbed when
the column breaks at the constriction, it's easy to read the highest temperature
reached by the thermometer. Shaking the thermometer gets the mercury
down due to inertia and ultimately drives it through the constriction so that it
rejoins into a single column.
5. Colours without dyes
The colours of nature are all around us and are produced by different aspects
of the interaction of light with matter. The most common is light interacting
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4. Physics at Work and Play 4
with coloured pigments. The reflection and absorption of light on a red flower
produces a red colour due to the pigments present in the petals. Some colours
in nature are produced by the break-up and interplay of white light. The sky
appears blue because molecules of air scatter blue colour more than the other
colours. Sunsets appear red because the light from the Sun passes through a
thicker layer of the atmosphere which scatters away most of the blue colour
leaving only the red/orange colours that reach our eyes. The colours of
rainbow are produced by break-up and total internal reflection of sunlight by
raindrops in the atmosphere.
The colour in oil films are produced by an entirely different process called
"interference of light" which is due to waves of light interacting with each
other. If the crest (peak) of one wave meets the trough (low) of another they
cancel each other – a process called "destructive cancellation". When the crest
of one wave meets the crest of another they reinforce each other and become
stronger – a process called "constructive reinforcement."
When diffused white light strikes an oil film on water it is reflected from both
the top surface as well as the bottom surface of the film. The film being very
thin and of non-uniform thickness, light reflected from the two surfaces
undergo constructive or destructive interference when seen from different
angles. But since white light is made up of several wavelengths, only light of a
particular wavelength or colour undergoes destructive interference when
reflected from certain regions of the film; the rest of the colours reach the eye.
As a result we see bands of colour in the film. The same principle applies for
colours seen on soap bubbles.
6. Ink dropper
Most of us have used a dropper to fill ink in a pen, pour a few drops of
reagent in chemistry lab, or put medicine drops in the eye. A typical dropper
consists of small glass tube with a narrow tip at one end and a rubber bulb at
the other. To fill ink or medicine in the dropper we squeeze the rubber bulb
and dip the narrow tip in the liquid. The liquid fills the tube when the bulb is
released. The dropper comes in handy when we need to measure only a few
drops of a liquid. How does it work? How does the liquid fill the tube?
Well, it's the atmospheric pressure that does the trick. The rubber bulb is
made of elastic material and so if the bulb is squeezed and then released it
regains its shape because of its elasticity. But after squeezing the bulb if we
close the narrow tip with a finger, and release the bulb it doesn't regain its
shape. What actually happens is that when we squeeze the bulb air inside it is
driven out, and when we release the bulb after closing the tip with a finger air
cannot come back in. Atmospheric pressure acting from outside does not
allow the bulb to regain its shape.
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5. Physics at Work and Play 5
If, instead of closing the tip after squeezing, we dip the tip in a liquid and then
release the bulb, atmospheric pressure acting on the liquid forces it into the
tube and also allows the bulb to regain its shape. Once in the tube the liquid
won't come out by itself because atmospheric pressure holds it back. But we
can bring the liquid out of the tube in controlled drops by gently squeezing
the bulb.
7. Golf ball dimples
The shape and size of a ball depends on the game it is played with. But in
most games the ball used has a more or less smooth surface, except a golf ball,
which has dimples on it. Most golf balls have between 300 and 500 dimples,
which have an average depth of about 0.254 mm. Why is the golf ball
dimpled? Let's find out.
Air exerts a force on any object moving through it. A ball moving through air
has a high-pressure area on its front side. Air flows smoothly over the
contours of the front side and eventually separates from the ball toward the
backside. A moving ball also leaves behind a turbulent wake region resulting
in lower pressure behind it. The size of the wake affects the amount of drag;
that is, the slowing action on the ball. The dimples on a golf ball helps reduce
the size of the turbulent wake region behind the ball by creating a thin
turbulent boundary layer of air that clings to the ball’s surface, which allows
the smoothly flowing air to follow the ball's surface a little farther around the
back side of the ball. A dimpled ball thus has about half the drag of a smooth
ball. Thus the dimples help the ball travel much farther when hit by the club.
A smooth golf ball hit by a professional golfer would travel only about half as
far as a golf ball with dimples does.
Dimples also help the golf ball convert spin into lift. A smooth ball with
backspin creates lift by warping the airflow such that the ball behaves like an
aircraft wing, making the air pressure on the bottom of the ball higher than
the air pressure on the top; this imbalance creates an upward force on the ball,
producing the lift. In case of a dimpled golf ball the pressure difference is
higher due to creation of a thin turbulent boundary layer, which increases the
lift and makes the ball go much farther than a smooth ball would go.
8. Ironing clothes
Cotton clothes are more comfortable than clothes made of synthetic fabrics.
But cotton fabrics have one disadvantage -- they crumple easily. Especially
after washing and drying cotton clothes become so wrinkled that you can't
wear them without ironing. But you can't iron a cotton shirt or cotton kameez
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6. Physics at Work and Play 6
without wetting it first, using a dry iron. Why do you need to wet cotton
fabric before ironing?
Polyester and nylon are synthetic fabrics that become soft below 100°C and
can be ironed smooth at a low temperature. Cotton fibre is made of cellulose,
which cannot be softened by heat. In cotton fibre the cellulose chains are held
together by weak attractive forces called "van der Waals" forces. When
cellulose absorbs water this attractive force becomes weak and the fibre
becomes soft; now it can be reshaped into any form, which it retains after
drying. In fact that is what happens when wet cotton fabric dries -- the fabric
retains its wrinkled shape. And that is why we need to wet cotton clothes
before ironing. When water is sprayed on the dry garment the fibres become
soft and can be stretched smooth. When the hot iron is moved over the wetted
cloth with pressure the wet fibres in contact with the smooth bottom of the
iron dry and set with a smooth surface, free of any wrinkles. In modern steam
irons steam rather than water is applied on the cloth, and produces the same
result.
9. Javelin throw
Javelin throw is a popular athletic event in which a metal or metal-tipped
spear is thrown for distance. The men's javelin is about 2.6 metres in length
and weighs 800 grams; the women's is about 2.2 metres in length and weighs
600 grams. Modern day javelins are made out of aluminium or graphite
composite.
Two major aerodynamic forces -- lift and drag -- act on a javelin in flight. Lift
is the force that keeps the javelin in the air, and drag is the force that opposes
the javelin's flight. Drag works against the javelin at any angle of flight but it
is the greatest as the angle of attack increases and more of the javelins surface
area is exposed. These two forces act on the javelin in a spot know as the
'centre of pressure', which is not fixed but can shift in relation to the centre of
gravity. When the centre of pressure is in front of the centre of gravity the
javelin remains tip up. When the centre of pressure moves behind the centre
of gravity the javelin tips down. So to reach the greatest distance the thrower
has to strike a balance between the two.
The key objective in javelin throwing is to throw as far as possible without
crossing the foul line. There has been much debate over what is the ideal
angle to throw the javelin at. Although no there is no consensus any angle
between 34-36 degrees is considered appropriate in calm conditions, but the
appropriate angle can shift anywhere from around 30 degrees to around 40
degrees depending on wind conditions. Throwing at a lower angle, by
exposing less of the javelin to air pressure, can reduce the drag, while still
enjoying an increase in lift.
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Although the throwing angle is important, it is not the only factor that
determines the distance of throw; there are three variables that determine it.
These three variables are the height of the throw, velocity at the release, and
angle of release. Velocity at release is the perhaps the most important factor in
javelin throwing.
10. Karate blow
In recent years, the ancient art of karate (from Karate-Do, a Japanese word,
literally translated as “the way of the empty hand”) has become quite popular
in India. Lakhs of children and young people are going for karate training as a
self-defence tactic. One of the common feats of a karateka (practitioner of
karate) is to break a pile of boards or bricks with nothing but a fist blow. How
do they achieve it?
The basic principle behind a karateka's performance is a rapid transfer of
momentum to the object being hit. The momentum of moving hand is mass
times the velocity. If the velocity is sufficiently high the momentum will also
be large. It therefore follows that the karateka should move his or her hand as
fast as possible in order to hit as hard as possible. An experienced karateka
can attain a velocity of 15 m/sec and a momentum of 45 kg m/s.
To be most effective, however, the momentum has to be transferred to the
object being hit in the shortest time possible, to maximise the impulse. That
means, the moving hand should hit the target without rebound that would
lead to loss of kinetic energy. If all the momentum of the moving hand stops
at one point, the momentum is transferred to the object being hit and is not
lost as kinetic energy. If a momentum of 45 kg m/s is transferred to the object
being hit in just 4 milliseconds (0.004s), the resulting force would be equal to
10,000 Newtons, which is much more than the structure of the board or brick
can handle.
To make a hit more effective the karateka also minimizes the area of the
striking surface to maximize the amount of force and energy transferred per
unit area. So he/she uses the side of the palm and not the whole palm to
strike, which increases the energy transferred per unit area almost nine times.
But it needs a lot of training and deep concentration for a karateka to break a
pile of boards or bricks with a fist blow.
11. Load and comfort
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8. Physics at Work and Play 8
If you have ever seen a pucca house under construction you'd have noticed
that the foundation of buildings up to four storeys are usually made much
broader than the wall thickness. Do you know why is it so?
Well, the foundation is made broader to make the load-bearing area as large
as possible. As a result the weight of the building is distributed evenly over a
wide area and the building does not 'sink' into the ground. All this has to do
with how much force is transferred per unit area to the load-bearing surface --
the more the surface area over which the force is distributed the less is the
pressure experienced.
We have many such examples -- the camel's feet, fibreglass moulded seats,
etc., where a larger area of contact reduces the pressure experienced. If we
look at the footpad of the camel we'll find it is quite broad, which
considerably reduces the force acting on the ground per unit area. As a result
the camel's feet do not sink in the soft sand.
Although hard, a moulded fibreglass seat feels comfortable because its
contours almost match our body contours and thus greatly increase the area
of contact and reduce the pressure points. The same is true of the track on
which a heavy battle tank moves; it also distributes the heavy weight of the
tank over a large area and the force per unit area experienced by the ground is
reduced considerably. As a result the heavy tank can move over soft ground
without sinking. The story of sadhus lying on beds of nail without feeling
pain can also be explained by the same principle. Although a single nail
would easily pierce through the skin because the entire weight of the sadhu's
body would act on a single point, when several dozen nails are used the
pressure felt at each point of contact would be much less because of the large
number of contact points. Similarly, if you prick a balloon with a sharp pin it
will burst. But if you make a 'bed' of several dozen pins and press the balloon
against it the balloon won't burst. Here, too, when several dozen pins are
used, the applied force is distributed over a large number of pin tips each of
which is insufficient to pierce the rubber membrane of the balloon.
12. Long jump
Many of you may have seen long jumpers in action at an athletic meet. After
running a short distance before taking off the athletes land into a pit filled
with fine sand. Each participant attempts to land as far from the take-off point
as possible. But if you observe closely you'll find that the long jumpers don't
just land a distance away; they appear to 'run' several steps through air after
taking off and before landing. Why do they do that?
If you thought it increases the jumper's speed through air you'd be mistaken.
The running action is done purely to maintain balance. The 'hitch-kick', as the
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9. Physics at Work and Play 9
running motion in air is called, stops the forward rotation of the jumper's
body that he gets when he springs into the air. Just before taking off, as the
jumper plants his foot on the jumping board, the motion of his lower body
stops for the fraction of a second when his foot is in contact with the board.
But his upper body continues to move forward, which makes him start to
rotate forward around his centre of gravity. If no corrective action were taken,
this rotation could send him toppling over and fall facedown into the sand.
This is prevented by the hitch-kick.
During the hitch-kick, jumpers hold each leg straight as it moves backward
and bent at the knees as it comes forward. This difference in leg position
causes the jumper's lower body to move forward. Similarly, the jumper's arm
movements during the hitch-kick push the jumper's upper body backward.
These body motions, which appear as running in air, neutralize the takeoff
rotation and allow the jumper to maintain an upright posture and get into a
better position for landing.
13. Mirage on a road
Deserts are extremely hot places where temperatures in summer can reach
50°Celsius. There are any numbers of stories of thirsty travellers in search of
water who keep moving attracted towards what looks like a distant lake or
pool of water, only never to find it. In city roads in summer you can see a
similar phenomenon; distant buildings and cars appear reflected on a puddle
of water on the road where there is none. What produces these reflections
without water?
What appears to be a lake or pool of water in a desert or a puddle of water on
a city road is actually an optical phenomenon called a 'mirage' that creates the
illusion of water; it is produced by layers of hot and cool air. Cold air being
denser than warm air bends light more. So as light passes from colder air to
warmer air it bends less, moving away from the direction of boundary
between the two layers. When light passes from hotter to colder air, it bends
more, towards the direction of the boundary.
Ordinarily hot air rises and the density of air decreases with altitude. But if
the ground surface is very hot, as in the desert or a city road in summer, the
air immediately in contact with the ground remains very hot while air above
it remains relatively cooler. When light from the sky or distant objects enters
the layer of the extremely hot air close to the ground at a shallow angle after
passing through the cooler layers of air above, it curves upwards, reaching the
eye of the observer from below. This produces the illusion of a shimmering
reflecting expanse resembling the surface of a body of water that does not
exist. In deserts, the mirages are actually images of the sky being refracted
back up from the hot air in contact with the hot sand.
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10. Physics at Work and Play 10
14. Physics of swings
Every child and even adults love swinging. In playgrounds swings are rarely
empty, with children vying with each other to get on to the swings. Little
children need a push to keep them swinging, but grown up children can keep
the swing moving and going higher and higher by a simple process of
'pumping'. How do they do it? How do the keep the swing moving without
anyone giving it a regular push? Let's see.
Swings are really a form of pendulum and so use the same physics concepts.
When you use your legs to make yourself stand up or squat on the swing you
are doing so by raising and lowering your centre of gravity, which generates
the extra movement. Pulling backward on the ropes raises your body,
decreasing the radius with respect to the support point, and thus increasing
your velocity. The same principle applies when you pump in a sitting position
by stretching and folding your legs. But the pumping would not work if not
done precisely at the right time in each cycle to synchronise with the natural
frequency of the swing.
Since a swing is basically a pendulum it's possible to calculate its resonant or
natural frequency using pendulum equations as follows:
f = 1/2p (g/L)0.5 where: g = gravity constant = 9.8 m/s/s for Earth, and
L = Length.
Note that the natural frequency of the swing is not influenced by the mass of
the person in it. In other words' it makes no difference whether a swing has a
large adult or a small child in it. If the swing is pushed, or pumping is applied
at the natural frequency of the swing it would resonate and its amplitude
would increase during each back and forth cycle.
15. Reducing friction
Whenever two surface rub against each other we encounter friction. Friction is
a force that resists the relative motion or tendency to such motion of two
bodies in contact and always acts in a direction opposite that of the motion.
Friction poses a real problem in smooth running of machinery – bicycles, cars,
fans, sewing machines – everything that have moving parts. Unless
something is done to reduce friction the moving parts become hot and wear
out fast. Fortunately, there are substances called lubricants, like grease,
lubricating oil, and graphite powder that, when applied as a surface coating
to moving parts, can reduce friction substantially. How do they do it?
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11. Physics at Work and Play 11
Usually, surfaces of machine parts that appear smooth and polished, have
irregularities -- little bumps and scrapes that can be so small that they show
up only under a powerful microscope. When two such surfaces in close
contact move in opposite directions these minute irregularities get caught on
each other and act to oppose the movement. That is friction. The job of a
lubricant is to fill up those tiny irregularities and allow the two surfaces to
slide over each other smoothly.
A wide range of substances – solids, liquids, and greases – are used as
lubricants, depending upon the purpose for which they are used. Lubricating
oils are easy to apply but cannot be used in places where they can flow out.
For applications such as fan bearings, and pump bearings and moving car
parts, grease is used. Grease lubricants have several advantages over oil
lubricants because they require less maintenance and do not need stringent
sealing of the lubricated parts. Solid lubricants include substance such as
graphite, molybdenum disulphide, Teflon, and boron nitride. They are useful
for conditions where conventional lubricants are inadequate such as in
applications where a sliding or reciprocating motion is involved; at high
temperatures where liquid lubricants typically would not survive; and under
extreme contact pressures.
16. Remotes
In cities many of us use remote controls for switching on and changing
channels on the TV, to operate a VCD or DVD player, to control an air
conditioner, or to lock or open cars. But all remote devices do not work in the
same way. For instance, remotes used for TV, VCR, DVD players or air
conditioners have to be pointed at the device being controlled. If the remote
is pointed away it doesn't work. But a car remote control works even if it is
not pointed at the car. Why this difference?
Remotes are primarily wireless devices, which use some kind of
electromagnetic waves to control a gadget kept at a distance. TV, VCR, DVD
player remotes and remotes used for air conditioners use a narrow beam of
invisible infrared waves for operation. The remote has an electrical circuit
that produces pulses of infrared waves from a light emitting diode (LED)
fixed at the front end of the remote device. The LED has a reflector or a lens to
produce a narrow beam that can be directed at the gadget being controlled.
The remote also has several buttons for different channels and other
operations.
When you press a button on the remote, a specific connection in the circuit is
completed. The chip in the remote senses that connection and knows what
button was pressed. It produces a Morse-code-like pulsed signal specific to
that button. The transistors amplify the signal and send them to the LED,
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12. Physics at Work and Play 12
which translates the signal into infrared light. The sensor in a TV, VCR, DVD
player, or air conditioner can see the infrared light and react appropriately.
Since the infrared beam is highly directional the gadget can respond only
when the beam is directed at its sensor. That is why it is necessary to point the
remote at the gadget being controlled.
Car remotes used for locking and unlocking car doors from a distance use
high-frequency (300 or 400 MHz) radio waves. The small unit attached to the
key chain is actually a small radio transmitter. When you push a button on
your remote, you turn on the transmitter and it sends a digitally coded radio
signal to the receiver fitted in the car, which is tuned to the frequency that the
transmitter is using. Once the car receiver senses the correct digital code it
provides power to the actuator that unlocks or locks the doors. Since radio
waves travel in all directions, a car remote need not be pointed at the car.
17. Sagging wires
If you have ever watched high-tension transmission lines that criss-cross the
countryside, you must have noticed that the wire cables strung across the
transmission towers are not stretched taut; they sag between the towers. But
when you see the overhead lines of electrified railway tracks they look
perfectly horizontal. Of course, they need to be perfectly horizontal;
otherwise it won't be possible for the pantograph of the electric locomotive to
be constantly in contact with the overhead wire. But why do transmission
lines sag but overhead electric traction lines do not?
When a flexible cable of uniform density and cross section is hung freely from
two fixed points it has a natural tendency to take the shape of a curve called a
"catenary" due to the effect of gravity. The cable sags because it has weight. A
sagging cable is stable; that is, it does not exert any sidelong force on the
suspension points. If the cable is not allowed to sag the towers could collapse.
Although a sagging cable presents no problem in power transmission, it is
certainly unacceptable for railway electrification. But making the wire taut
could make the structure unstable. Railway engineers found a simple solution
to the problem. They devised a structure consisting of an upper structural
wire in the form of a shallow catenary, to which a lower conductive contact
wire is attached with short suspender wires of different lengths. In this
arrangement, since the upper structural wire is allowed to sag, it exerts no
extra force on the pillars. By adjusting the tension in various elements the
conductive wire is kept horizontal -- parallel to the centreline of the track,
allowing uninterrupted contact with the overhead pantograph of the train.
18. The starting block
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If you have ever seen athletes at the start of a sprint event you may have
wondered why they crouch low with their hands touching the ground and
feet firmly set against what look like inclined foot pads. The inclined footpads
are 'starting blocks' which are used by the athletes to get off to a good start in
a race. By starting from a crouching position, and pushing against starting
blocks the sprinters are able to accelerate better.
Early sprinters used to dig holes in the track in which to place their feet when
starting, to get an extra push at start. Track coaches have been striving for
years to develop some kind of technique to improve their sprinters'
performance. Research has produced some staggering advances, but none has
had a more significant effect than the starting block, especially after the
advent of synthetic track surfaces, where digging holes was out of question.
Blocks were introduced in the late 1920s and were first used at the 1948
Olympic games in London.
Starting blocks are usually made of aluminium and have a centre rail and
slotted angles to firmly grip the blocks. They are made adjustable to four
different angles and are fitted with special thick rubber to take in spikes of the
athlete’s boots.
But merely using starting blocks cannot improve the performance of a
sprinter unless the blocks are set properly and the sprinter takes up the right
posture. Technically, the distance between the front block and the starting line
should be two foot-lengths of the athlete. The rear block is to be placed
another foot length behind the front block. Spacing can be adjusted based on
comfort, existing strength levels, etc. For best start, the front knee angle
should be between 90 and 110 degrees, while the rear leg angle should be
between 120 and 135 degrees.
19. Swinging the ball
Swing is one of the most important weapons in the arsenal of a fast bowler.
After leaving the bowler's hand the ball at first appears to come straight at the
batsman but then swerves towards or away from him, often forcing him to
'nick' the ball and get caught behind. How does a bowler swing the ball?
Swing is easily explained by physics. When the ball moves through air it cuts
through the air which moves around it, but the velocity at which the air
moves depends on the nature of the surface. If the surface is smooth and
shiny the velocity of air moving in contact with it would be fast, but air
moving in contact with a rough surface would not move as fast. The key to
making a cricket ball swing is to cause a pressure difference between the two
sides of the ball. Since the air pressure depends on the flow of air over each
side of the ball, if one side of the ball is made rough then air flow on that side
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14. Physics at Work and Play 14
is reduced but air flow on the other, shiny side would be fast. As a result,
according to Bernoulli's principle, the air pressure on the shiny side is
reduced and the ball would swerve towards the shiny side. By shining the
ball on one side, and carefully positioning the seam, which runs around the
ball, bowlers can make it curve through the air as it approaches the batsman.
Bowlers are allowed to polish the ball by rubbing it with cloth (usually on
their trouser legs) and applying saliva or sweat to it. Any other substance is
illegal, as is rubbing the ball on the ground. It is also illegal to roughen the
ball by any means, including scraping it with the fingernails or lifting the
seam.
20. The cutting edge
It is common knowledge that cutting tools should be sharp. The carpenter
always keeps his tools sharp, as does the cobbler, barber, or the man in the
meat shop. A sharp knife cuts better than a dull knife. It is difficult to cut
something with a dull knife; you need to apply more pressure than you
would need if you use sharp knife. Why is it so? Why doesn't a dull knife cut
well?
In simple words, all this has to do with how much force is transferred per unit
area to the surface being cut. If the knife-edge is sharp the total area of the
cutting edge (area of contact) would be much smaller than the total area of the
cutting edge of a dull knife. So, if equal force is applied on both the knives,
the force per unit area experienced by the surface being cut with the sharp
knife will be much more than the force experienced by the surface being cut
by the dull knife. Obviously, the former will cut better than the latter.
Dull knives not only lead to excessive use of force to cut materials, they also
increase the chance that the blade may slip and the force transferred to an
unintended destination such as the user or another person and cause injury.
The same logic applies to other sharp objects like needles, nails and pins – the
sharper the better.
21. The fictitious force
If you are travelling in a car and the car swerves around a corner, you'd find
yourself pushed against the outer edge of the car. It appears that some unseen
force is acting to push you against the side of the car. Commonly this unseen
force is termed as 'centrifugal force', a force that tends to move objects away
from the centre in a system undergoing circular motion. It also keeps the
water in a whirling bucket from spilling or keeps roller coaster riders from
falling out when coaster 'loops the loop'. However, it is not a real force but an
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15. Physics at Work and Play 15
apparent force, equal and opposite to the centripetal force, that draws a
rotating body away from the centre of rotation; it is caused by the inertia of
the body.
Centrifugal force can be explained in terms of Newton's laws of motion. As
the car changes direction the passenger's inertia resists acceleration and
change in direction, keeping the passenger moving with constant speed in the
same direction. But since the car turns it appears that the passenger is being
pushed against the side whereas actually the passenger does not move
toward the side of the car; instead, the car curves in to meet the passenger.
Although considered a fictitious force, centrifugal force has many
applications. Centrifuges and ultracentrifuges are used in science and
industry to separate substances by their relative masses. Centrifugal
governors use spinning masses to regulate the speed of an engine by
controlling the throttle. Centrifugal force can be used to generate artificial
gravity in space stations. The oblate shapes of the planets Jupiter and Saturn
are explained as due to centrifugal force created by their rapid spins.
22. A question of steering
When you ride a bicycle, or drive a motorbike, or drive a car you need to steer
it to turn it in any desired direction. In bicycles, motorbikes and two-wheelers
in general the steering is done with a handle bar, but four-wheelers use a
steering wheel for the purpose. Why are they different?
To find an answer this question we've to first understand how a vehicle is
steered. A two-wheeler such as a bicycle or a motorbike is steered primarily
by turning the front wheel (along with a slight tilting, called banking, in the
direction of the turning). Since the front wheel is fixed directly under the
handle bar that can be turned around a vertical axis fixed to its centre, the
front wheel can be turned simply by turning the handle. When a torque is
applied around the middle of the handle bar the front wheel turns.
In a four-wheeler not one but both front wheels need to be turned for steering
and here a system of rack and pinion is used. For a car to turn smoothly, each
wheel must follow a different circle. Since the inside wheel follows a circle
with a smaller radius, it actually makes a tighter turn than the outside wheel.
And to do this a rack and pinion mechanism is used with a steering wheel.
The pinion gear is attached to the steering shaft. When the steering wheel is
turned, the gear spins, moving the rack. The tie rod at each end of the rack
connects to the steering arm on the spindle, which turns each of the wheels.
On most cars, it takes three to four complete revolutions of the steering wheel
to make the wheels turn from lock to lock (from far left to far right).
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23. Tractor wheels
We see many types of four-wheelers on the road – cars, buses, trucks, tankers,
tractors, and many others. Among these there is something peculiar about
tractors – their huge rear wheels, sometimes as large as 1.6 metres across. All
the other four-wheelers have all the four (sometimes more) wheels of the
same size. Why do tractors have large rear wheels?
To answer this question we have to look at the purpose for which tractors are
used. Tractors are mostly used for farming and that means they have be
driven over soft, often muddy, soil. The large rear wheels with large surface
area distribute the weight of the heavy tractor over a large area and thereby
prevent the tractor from getting bogged down in wet field. The deep and
wide treads also provides a firm grip in mud preventing the wheels spinning
freely as would happen with ordinary car wheels.
Also the large rear wheels means larger area of contact with ground than a
small wheel. So a large wheel provides more traction power, which is
necessary for the tractor to pull, farming implements such as ploughs and
tillers through the dry or wet ground. Larger diameter means greater pull. In
India, tractors are also used for hauling a variety of goods and here also the
large rear wheels provide better hauling power.
The front wheels of tractors are small because they are used mainly for
steering, and large wheels are harder to steer. Small front wheels also give the
tractor operator a clearer view of the rows through which it is working and
possibly better turning capability. Of course, many of today's tractors come
with all wheels of the same size, but they are used for different purposes. In
many foreign countries four-wheel-drive tractors are also available with
power-assisted steering.
24. Tyres with tread
Millions of vehicles ply on the roads every day and all use wheels with air-
filled rubber tyres. Air-filled tyres make the ride comfortable by cushioning
the jerks caused due to bumps on the road surface. They also provide a high-
friction bond between the vehicle and the road surface to improve
acceleration and handling.
Although vehicle tyres come in a wide variety of shapes and sizes they have
one thing in common – all come with treads cut around their circumference.
A tyre is considered safe as long as the treads remain visible; tyres with
completely worn-out treads are considered unsafe. How do the treads on a
rubber tyre make it safe?
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17. Physics at Work and Play 17
Rubber tyres offer good grip on the on the road because of friction that arises
due to adhesion between surfaces. Increase in contact area between two
surfaces increases the frictional forces. If an elastic material like rubber is used
the real contact area further depends on the load pressing the two surfaces
together. If the surface is grooved, as in a tyre with treads, the increase in
contact area with load is much more compared to a surface without grooves.
The design of the grooves and ridges of the tread affect the amount of the
deformation, and hence, the friction or grip on surfaces.
Besides increasing road grip, treads also cool the tyre while running at high
speeds, and provide a safe margin of rubber before the complete tyre wears
out. In wet conditions they provide ducts through which the water is
squeezed out. This in turn helps the tyre have better grip on wet roads.
25. Voltage vs current
We use all kinds of electrical appliances. Some work on batteries, others run
on mains power. We know that mains power is dangerous because the
voltage is high -- 220 volts for domestic supplies. If we accidentally touch a
live wire we get a shock. But we never get a shock if we touch the terminals
of a 6-volt battery although if we connect its two terminal with a wire the wire
gets hot. Similarly, transformers used for welding works at only 12 volts and
won't give a shock, but it can melt steel for welding. Why is it so?
To understand why a low voltage doesn't give a shock but can produce
enough heat to melt metal it is necessary to distinguish between voltage and
current. Voltage is a term used to designate electrical pressure or force that
causes current to flow. It is like the height of a waterfall; the difference
between the top and bottom levels represents the voltage. If the water drops
from a low height it would be analogous to a low voltage. When the water
falls from a higher level the force of the fall is much greater than the force of
fall from a lower height. In an electrical circuit it is this force that we feel as a
shock. If the voltage is low we don't feel any shock.
Current is like the volume of water flowing in the waterfall. Even if the height
is large the water flow may be just a trickle, which won't produce any useful
work. If the volume of water flow is large, even with a lower head it can
produce useful work.
Heating of an electrical conductor is proportional to the square of current
flowing through it (Heat produced = I2R), so the wire connecting two terminal
of a battery get hot because more current flows through it. The transformer
used for welding is a step-down transformer, which reduces the output
voltage to 12 volts from 220 volts of the mains. But, since the product of
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18. Physics at Work and Play 18
voltage and current remains constant, stepping down the voltage increases
the current flowing through the circuit several folds, which is essential for
welding.
26. Of friction and walking
If you have ever walked on ice, you might know how easy it is to slip and fall.
Ice is very slippery because it offers little friction. Similarly, if you happen to
step on a banana peel you'll meet the same fate, because the banana peel also
reduces friction and makes the road below your feet slippery. But why can't
we walk on a slippery surface? Let's find out.
Usually, even surfaces that appear smooth have irregularities -- little bumps
and scrapes that can be so small you can’t see them. Similarly, the sole of our
feet or shoes also have little bumps and scrapes. Friction is caused by these
irregularities getting caught on each other as two surfaces rub together.
Friction is a force that always acts in the opposite direction of the object’s
motion. Friction is necessary for walking.
When you walk, with each step you plant your foot firmly on ground and
push against it, thereby pivoting your body around that foot to move
forward. As you plant the other foot on the ground in front of you, the
ground exerts a force back up your leg and you rise up on that foot and move
another step forward. Every time you put your foot on ground it is held in
place by friction, which makes pushing easier. But if there is no friction your
foot cannot hold on to the ground and cannot push against it. The result? You
slip and fall.
Some things like a highly polished floor and ice don’t have many
irregularities to get caught on, so there is little friction. Without the friction,
you slip. On a concrete or paved road surface there are many rough spots for
your shoes to get caught on so you don’t slip. But a banana peel acts as a
blanket, smoothening out the rough spots, and makes you slip when you step
on.
In the language of physics, the 'required friction coefficient' represents the
minimum friction needed to support walking, while the 'available friction
coefficient' represents the maximum friction that could be supported at
interface between the shoe and floor without a slip. When the required
friction for an activity exceeds the available friction at the interface, you are
more likely to slip.
27. Water as fire extinguisher
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19. Physics at Work and Play 19
It is rightly said that things that burn never return. Fire is a great destroyer. A
badly managed fire can raze buildings and burn down hundreds of hectares
of forestland in no time. While prevention is the best course, once a fire starts
the best option is to extinguish it as fast as possible, and there are few
substances as efficient in killing fire, except electrical or oil fire, as water.
Although water is not directly used to extinguish oil fires the foam used to
extinguish such fires is mostly water-based. What makes water such an
efficient extinguisher of fire?
For a fire to occur, there must be available oxygen, a supply of fuel, and
enough heat to kindle the fuel. Therefore, the three basic ways of
extinguishing fire are to smother it, to cut off the fuel supply, or to cool it
below the flammability temperature.
Fires involving solid materials, such as wood, paper, straw, textiles, coal, etc.,
are the most common types of fire and the best way to control such fires is to
bring down the temperature of the burning material quickly, which water
does most efficiently. The property that makes water an efficient fire
extinguisher is its capacity to absorb large amounts of heat. In fact, water has
the highest specific heat capacity of any known chemical compound, as well
as a high heat of vaporization. That means, water absorbs the largest amount
of heat per gram for every degree rise in temperature and also for reaching
the boiling point and thus cools fast and puts out a fire.
28. Laser printer
Ever since the German inventor Johannes Gutenberg invented movable types
for printing in the mid-1450s that revolutionised written communication, the
technology has come a long way. Movable types have given way to linotype,
offset, intaglio, and flexi printing. Today personal computers allow users to
print a document at home or in office. Home printers even allow one to get
colour prints of one's favourite photos taken with a digital camera.
Home and office printers come in three basic types: dot matrix, inkjet and
laser printers. The terms dot matrix and inkjet printer are very descriptive of
the processes at work. The first prints the alphabets as a combination of dots
imprinted by a matrix of pins impacting against an inked ribbon. Inkjet
printers put an image on paper using tiny jets of ink. But the term laser
printer is a bit more mysterious – how can a laser beam write letters and draw
pictures on paper?
Actually the laser beam does not do the writing. The primary principle at
work in a laser printer is static electricity, which is used to charge a polished
surface of an insulated drum coated with a photoconductive material. The
drum is first given a total positive charge by an electrically charged corona
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20. Physics at Work and Play 20
wire. The unique property of the drum coating is that the charge can be
reversed by exposure to light. When the "print" command is given to the
printer, a tiny laser beam "draws" the letters and images to be printed as a
pattern of electrical charges – an electrostatic image – on the charged drum.
At the regions where the laser beam strikes the positive charge on the drum is
neutralised and the characters imprinted on the drum gets a negative charge
with respect to the rest of the drum. When a positively charged toner is
applied on the drum it sticks only to the negatively charged areas; that is, the
characters imprinted by the laser beam on it. The toner pattern is the
transferred to paper, which is then heated to fix the toner permanently. Thus
in a laser printer the laser does not actually do the printing, but only "draws"
an electrostatic image which is turned into visible characters on paper by the
toner.
29. Is it vertical?
In any building construction work the walls have to be perfectly vertical. This
is necessary to ensure that the load falls vertically on the ground. A wall tilted
from the vertical would collapse under load if not supported on the tilted side
by buttresses. But how does one ensure the verticality of a wall? Masons do it
using a simple device called the "plumb line". The plumb line employs the
law of gravity to establish what is "plumb"; that is, what is exactly vertical, or
true.
The plumb line is basically a conical metal weight attached to a string which
hangs vertically in Earth’s gravitational field if let free. When freely
suspended, the hanging string is directed exactly toward the Earth's centre of
gravity and can be used as a vertical reference line. The line has in every point
the same direction as that of the force of gravity of the Earth; thus, an object
dropped on the surface of the Earth tends to follow this line. To use the tool,
the string is fixed at the point to be plumbed. The weight, or bob, is then
allowed to swing freely; when it stops, the point of the bob is precisely below
the point at which the string is fixed above. When the plumb line is
suspended from a wall under construction the bob should just touch the
lower end of the wall if the wall is perfectly vertical.
The plumb line is also used to transfer points from a height on to ground. Of
course, the transfer can be done without using a plumb line, by mere eyesight
if the observer looks vertically straight down, but that is not always possible.
Any shift in the eye position from the vertical would introduce a “parallax”
error and the point marked on the ground would not be directly below the
point at a height but away from it.
30. The spade
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21. Physics at Work and Play 21
The spade is a common gardening implement used for a variety of purposes.
The spade consists of two parts; the blade, of plate-iron, and the handle, made
of tough wood. The wooden handle ends in a crosspiece, usually forming a
kind of loop for the hand. The blade consists of two parts; the plate, by which
the soil is cut and carried, and the tread, which is a piece of strong iron fixed
on the upper edge of the blade, to receive the impulse of the foot of the
operator. Spades are manufactured of different sizes, and usually with a flat
blade. In gardening, a spade is used to dig or loosen ground, or to break up
clumps in the soil. It is sometimes considered a type of shovel.
A spade is used as a lever of the first class and also as a lever of the third class.
When it is used as a digging tool, the load acts at the end of the flat blade, the
middle of the blade acts as fulcrum and the force of the hands acts on the
handle at the other end. Depending on the size of the blade and the handle a
spade can give considerable mechanical advantage, which can be used for
extracting dead or cut tree stumps from the ground. When a spade is used lift
soil or rubble, the spade acts as a lever of the third class -- the load acts on the
blade, the handle acts as fulcrum and the hand holding the middle exerts the
force to lift the load. Although the mechanical advantage of a third class lever
is always less than 1, for least effort the lifting force should be applied as far
away as from the fulcrum; that is, the lifting hand should be placed as near
the blade as possible.
31. Carrying a load
Throughout history men and women of all ages have carried goods, food,
supplies and arms for the purpose of survival. Today, despite many
technological advances, this basic form of human-powered transportation
remains an indispensable resource for many occupational tasks and activities
of daily life.
A person can carry a load in many different ways -- on the back, on one hand,
on both hands, on shoulder, or on the head. In studies on the energetics of
different modes of carrying load it has been found that the metabolic cost of
carrying large back-supported loads was almost twice that of carrying large
head-supported loads for the same load at the same walking speed. The
rucksack method of carrying load on the back was found to be the more
economical than carrying load in hand, in terms of energy expenditure.
It is easy to understand why carrying a load on the head needs the least effort
compared to any other method. From physiological point of view, the best
way to carry a load would be one that does not put undue strain on the body
structure to sustain balance. Obviously carrying a load on the head would be
the most comfortable because here the load would act vertically and would
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22. Physics at Work and Play 22
not upset body balance, although it would raise the centre of gravity of the
body as whole, making it prone to fall. Carrying too much weight on the head
also may be dangerous; it could cause cervical spinal cord injuries.
On the other hand, carrying load on the shoulder or in hand puts strain on the
body structure in the form of a bending force that the body has to counter
constantly to maintain balance. This has also been proved in studies, which
showed that hand carriage caused marked side bending of the trunk and poor
posture. Carrying the load with the hands by the side proved to be the worst
in terms of physiological efficiency. Carrying a bag on one shoulder, by
putting constant stress, was found to lead to posture that might predispose to
back pain.
32. Reinforced concrete
Reinforced cement concrete (RCC) is a standard building material used in
building construction of all types. Brick buildings may have walls made of
bricks, but the roof is always cast in RCC. The material is called "reinforced"
cement concrete because it contains steel reinforcements in the shape of thin
bars embedded in the concrete.
Typical concrete mixes have extremely high resistance to downward
compressive stresses (about 21 million pascals); however, any appreciable
stretching or bending (tension) will break the microscopic rigid lattice
resulting in cracking and separation of the concrete. For example, a
foundation of cement concrete without steel reinforcement would be able to
take the load of a large building without failing, but if a roof slab or a beam is
cast without reinforcement it would easily give way under heavy load. The
steel reinforcement embedded in RCC helps the concrete withstand high
tensile loads and thus prevents the concrete structure from breaking up under
heavy load.
When a load is placed on a slab or a beam supported on two ends, the load
induces compression on the upper part and tension in the lower part of the
member. So the reinforcement is also placed in the lower part; that is, close to
the lower surface of the slab or beam. However, if the structural member is
supported only on one end, then loading produces tension in the upper part
of the member. So the reinforcement is placed near the upper surface of the
member.
33. Why do we need earthing?
Domestic electricity supply in India is 220 volts AC. Any accidental contact
with a current carrying conductor, due to a faulty electrical gadget or a short
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23. Physics at Work and Play 23
circuit at 220 V AC can be fatal. Hence a safeguard is provided in AC circuits
in the form of a third conductor called the "earth".
The main objective of earthing is to provide an alternative path for any
leaking current to flow to the ground so that it would not endanger the user.
The earthing of an electrical installation not only provides protection for
persons against the danger of electric shock, but also maintains the proper
function of the electrical system. The Earth always maintains a zero potential -
- it is neither positively nor negatively charged. So, when a faulty electrical
circuit is connected to earth the leaking current can safely flow to the ground
through the earth wire because it offers much less resistance compared to the
body of the user, thus causing no harm.
The 'earth' terminal of an electrical circuit is made up of a conducting wire
connected to a metal conductor buried in the ground. All exposed metal parts
of an electrical installation or electrical appliance need to be connected to this
wire. This is done by using 3-wire conductors and a three-pin plug and
socket. All electrical outlets with a 3-pin socket have an earth connection. In a
3-pin plug the longest prong, which is connected to the green wire, is the
earth connection. The other end of the green wire is usually connected to the
metal body of the electrical appliance. The earth prong is made longer so that
in case of any current leakage the earth connection is established before the
faulty gadget is connected to the mains and an accident can be prevented.
34. Pumping water
Every day, millions of water pumps deliver water from wells to homes, farms
and businesses. Conventional hand pumps or centrifugal pumps can lift
water from a well only if the water level is less than 10 metres below the
surface. This is because these pumps depend on the pressure of the
atmosphere to lift water. The low pressure created inside the pump by the
piston or impeller makes the normal pressure of the atmosphere push the
water up through the pipe. You can think of it as a long straw you use to suck
soft drinks. As you suck on the straw, a low pressure is created in the straw
above the liquid and normal atmospheric pressure pushes the liquid up the
straw. Consequently, the height that you can lift water with a hand pump or
centrifugal pump relates to the height of a water column the atmospheric
pressure can support. This height is about 10 metres because the atmospheric
pressure is able to support a water column about that high. So a hand pump
cannot lift water from a depth greater than 10 metres. Then how do you lift
water to the top of multi-storeyed buildings?
The answer is simple. While there is a limit to the depth from which
conventional pumps can lift water, there is no limit to the height to which a
pump can push water up. So, if the depth is greater than 10 metres, water can
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24. Physics at Work and Play 24
be lifted if the pump is placed near the bottom of the well, submerged in
water. Such a pump, called a submersible pump, can lift water from depths
of more than 10 metres because they actually push the water up from the
bottom of the well. A typical submersible pump is characterized by a long
cylindrical shape that fits inside the well casing. The bottom half is made up
of a sealed pump motor that is connected to the aboveground power source
and controlled by wires. The pump itself is made up of a stacked arrangement
of impellers that drives the water up the pipe to the plumbing system. Here,
depending on the power of the pump, water can be lifted to almost any
height.
35. Sticking them together
In our daily life we use a variety of substances to stick things together. Glue,
Fevicol, Cellotape, and many others are used as adhesives. But how do they
work; how do they keep two surfaces from coming apart once stuck together?
Well, it all has to do with some kind of bonding.
One thing that adhesives have in common is that they're made of polymers,
which are chain-like molecules. A good adhesive has excellent properties of
adhesion (the ability to stick to the surfaces to which it's applied) and
cohesion (the ability to stick to itself). The bonding can be of three kinds --
mechanical bonding, physical interaction, and chemical interaction.
Mechanical bonding involves some kind of "anchoring". The adhesive flows
into microscopic pores in the two surfaces and hardens after drying such that
it keys into the surfaces and forms a strong surface bond to hold them
together. Starch glue, and Fevicol are typical examples of this kind of
adhesive.
Bonding by physical interaction involves weak intermolecular attraction
called van der Waals force between the materials being bonded and the
adhesive. Here, the adhesive has a low surface tension and easily "wets" the
surfaces being bonded, which stick together due to van der Waals forces.
Adhesives used to bond smooth surfaces like steel and glass belong to this
class.
Some adhesives come in two parts -- a resin and a hardener or catalyst -- that
are mixed just before use. After application the mixed adhesive hardens by
chemical reaction. Epoxy-based adhesives such Araldite belong to this
category.
Pressure-sensitive adhesives stick to a surface when pressure is applied. Most
adhesive tapes use a pressure-sensitive adhesive. The tape looks smooth, but
it's not. Its adhesive coated side has tiny pits in which air bubbles get trapped
when the tape is applied to a surface. When pressure is applied air escapes
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25. Physics at Work and Play 25
from the tiny bubbles, which then act like minute suction cups holding the
tape strongly to the surface.
36. Engines without spark plugs
Motor vehicles that use fuel are primarily of two types – those which run on
petrol or CNG and those that run on diesel. In both the expansion of hot
gases provides power to the piston that drives the engine, but there is basic
difference between the two. In petrol and CNG driven vehicles spark plugs
are used to ignite the fuel mixture in the engine cylinder to provide the
power. But in a diesel vehicle no spark plugs are used. How is the fuel burnt
in a diesel engine?
In a four-stroke petrol or CNG engine the fuel and air is first drawn into the
cylinder, which is then rapidly squeezed by the piston into a small volume
(about 1/10th the original volume) that heats up the mixture. At the end of
the compression stroke the heated mixture is ignited by a spark and the
expanding hot gases push the piston down, providing the driving force.
Here, the fuel and air mixture is expected to wait until it's ignited at the
proper instant by the spark plug. That's why gasoline is formulated to resist
ignition below a certain temperature. The higher the "octane number" of the
gasoline, the higher its certified ignition temperature.
A diesel engine doesn't use spark ignition. Instead, it uses the high
temperature produced by extreme compression of air to burn the fuel. When
pure air is rapidly squeezed up to 1/20th the original volume, it becomes so
hot that it can ignite the fuel. Thus when diesel is injected into the cylinder at
the end of the compression stroke it bursts into flames and burns quickly in
the superheated compressed air. The rapidly expanding hot gases push the
piston to drive the engine.
37. Potholes
Every year during monsoon the rains play havoc with city roads. After a few
days of rain most city roads show up numerous potholes that keep growing
in size as the days pass, making driving a nightmare for motorists. How do
these potholes appear?
The appearance of pot holes have something to do with the material road
surfaces are built of, and simple physics. They appear only on asphalt roads
but never on concrete surfaced roads, for the simple reason that in concrete
the aggregates (stone chips) are firmly bound in a cement matrix, which is
impervious to water. And concrete can withstand large compressive load
without fracturing. But in an asphalt surface the aggregates are weakly
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26. Physics at Work and Play 26
bonded by asphalt, which deteriorates in contact with water, making the
surface vulnerable to break up.
A pothole usually begins as a tiny crack on the road surface. After rain, if
water accumulates on the surface, water seeps through the crack, and the
asphalt below the surface starts losing hold of the stone chips, which come off
gradually. The process is hastened when vehicles pass over the damaged
surface. Water forced out of the treads of the tyres moving vehicles act as
high-pressure jets, dislodging more chips out of the weakened surface and the
pot hole becomes bigger and bigger. However, if rainwater is not allowed to
accumulate on the road development of potholes may be prevented, as it is
the combined effect of accumulated rainwater and action of moving vehicles
that produce potholes on roads.
38. Screwdriver
Screwdriver is an essential ingredient of any toolbox. As the name implies, a
screwdriver is used to drive a screw into a surface or take it out. Sometimes
screws are inserted and fixed into threads cut in a surface, or used with a nut
to fix things. But in all these actions the screw has to be turned to fix it or
unfix it.
Screwdrivers come in many sizes and the size of the screwdriver used would
depend on the job to be done. A watchmaker can do with a very small
screwdriver to fix tiny watch screws, but a carpenter would need a long
screwdriver with a large handle for joining wooden pieces or fixing objects to
wooden frames. A watchmaker's screwdriver would be useless for the
carpenter and so would be a carpenter's screwdriver for a watchmaker. Why
this difference?
A screwdriver makes use of the lever principle for turning and fixing screws.
The tip of the screwdriver fits snugly on the head of the screw to be driven.
When the screwdriver is to be used, equal and opposing parallel forces, which
form a couple, are applied to turn it. Turning the tiny screws used in watches
needs very small force that can be applied by twirling the thumb and the
forefinger. Here, the diameter of the handle of the screwdriver is very small
and so is the lever advantage. But driving larger screws into wood needs
much stronger force, which cannot be provided by a small screwdriver. Here
a larger screwdriver with a thicker handle is required. A handle with a large
diameter not only gives a much higher lever advantage than a watchmaker's
screwdriver but also provides a stronger grip, making it possible to apply
much stronger opposing forces to turn the screw.
39. Propeller vs jet engine
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27. Physics at Work and Play 27
Since the American brothers Orville and Wilber Wright flew their first
heavier-than-air machine in 1903, air travel has come a long way. In its first
powered flight, Wright brothers' "Flyer", which flew on propeller power,
remained in air only for 12 seconds, and covered a distance of about 40
metres. Today's jet airliners can fly much faster than any propeller-driven
aircraft and can remain in air for more than a dozen hours, covering more
than 12,000 kilometres non-stop. Jet aircraft can also fly at much higher
altitudes than propeller-driven planes can. How do they work at altitudes
where air pressure is less than one-fifth at sea level? Let's find out how.
A propeller-driven aircraft makes use of Bernoulli's principle both for lift and
forward motion. The aerofoil shape of the wings makes the air over the top
move faster than the air under. Slower air has more pressure, so there is a net
upward thrust on the aerofoil, which produces lift. The blades of a propeller
act as rotating wings, and produce force through application of both
Bernoulli's principle, generating a difference in pressure between the forward
and rear surfaces of the airfoil-shaped blades. But a propeller's performance
suffers as the blade speed exceeds the speed of sound. That is why aircraft
with conventional propellers do not usually fly faster than Mach 0.6; that is,
faster than 60 percent of the speed of sound.
Unlike propellers, jet engines work well at high speeds and jet aircraft can fly
at speeds greater than the speed of sound. A turbojet engine is a type of
internal combustion engine. It works by first compressing incoming air with a
series of fan-like blades. Fuel is then mixed with the compressed air and the
mixture ignited. Finally, the high-energy gases and hot air is ejected at high
speed out of the rear of the engine, which pushes the aircraft forward
according to Newton's third law of motion. Most modern jet engines are
actually turbofans in which a large fan attached to the front end of a turbojet
engine is used to supply supercharged air to not only the engine core, but to a
bypass duct, which increases the efficiency.
40. Cutting glass
Glass is a unique material. It has the stiffness and brittleness of crystals but
lacks their large-scale regularity of structure. Glass is an amorphous
substance; with no regularity in the way their molecules are arranged in the
solid. Strictly speaking, glass is not a solid but a highly viscous liquid, but at
the same time it is very brittle. If you take a regular piece of windowpane
between your hands and try to break it in half by bending, it appears quite
stiff. If you apply enough force you can break the pane into two or more
irregular pieces. But, then, how do carpenters and photo framers cut glass
sheets with such ease -- just by making a thin scratch and applying a little
pressure?
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28. Physics at Work and Play 28
Actually, glass isn't really 'cut' in the normal sense of the word, but is only
subjected to a controlled break. Since glass has no crystalline structure it has
no cleavage planes (like gemstones, for instance). Also, since glass is equally
strong in any direction, you normally won't have to worry about direction of
grain like carpenters do. To cut a sheet of glass you first create a fine crack on
the surface by scoring with a diamond tip and then applying pressure in the
opposite direction. When you bend a glass sheet, you stretch one surface
while compressing the other and create a tensile stress on the stretched
surface. If there is a crack on the stretched surface, glass breaks along the
crack. By scoring the surface of the glass with a diamond stylus, you can
create that crack and control exactly where that lapse in tensile strength will
occur. Scoring disrupts the surface integrity along a thin line along which the
break occurs. This happens because glass is brittle and cracks can travel
through them easily. Once the crack starts to grow things go from bad to
worse. The crack becomes sharper and the stress increase at the tip becomes
larger and larger. The crack tip propagates through glass at roughly the speed
of sound and results in a clean break.
41. Badminton shuttlecock
Badminton is a popular game played with a racquet, which consists of a
handle and an oval frame with a tightly interlaced network of strings. But
unlike other games played with racquets like tennis and squash, badminton is
played with shuttlecocks -- a lightweight, open conical-shaped object made by
sticking 16 goose feathers into a hemispherical piece of cork (called the
bumper). Also known as a 'birdie', 'cock', or 'shuttle', shuttlecocks are high-
drag projectiles unlike the spherical balls used in other games; they encounter
high air resistance in flight. Yet, badminton is the fastest racquet sport in the
world with shuttles known to reach speeds of up to 332 km/h. How does the
odd-shaped shuttle fly so fast and why does it always turn around and hit the
racquet bumper first?
A spherical ball, being symmetrical in shape does not experience any torque
while moving through air. But a shuttlecock is symmetrical only around its
long axis and experiences least air resistance and torque only if it flies bumper
first through the air. In any other orientation, it experiences significant air
resistance and torque while moving through air. This is because the
shuttlecock's centre of pressure -- the point at which the overall pressure force
effectively acts -- isn't located at its centre of mass, which lies in the middle of
the cork bumper. It is this torque that quickly turns the shuttlecock around in
the air immediately after being hit by the racquet and makes it fly through air
bumper first.
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29. Physics at Work and Play 29
Being lightweight, a shuttlecock can be quickly accelerated to very high
velocities by applying the same force that would accelerate a heavier tennis
ball to a much lower velocity (F = m x a). With its bumper flying ahead of its
feathers, the shuttlecock has dynamic stability. If it turns in any direction, air
pressure immediately turns it in the opposite direction, thus maintaining its
orientation and stabilising it during flight. This aerodynamic stabilising effect
also flips the shuttlecock quickly after each hit and then keeps it flying
bumper forward until the next hit.
42. Xeroxing
The Xerox machine is standard equipment found in almost all offices. It is
used for making copies of documents including text and images. To make
copies you have only to place the document to be copied face down on the
glass sheet on the top of the machine and press a button. The machine does
the rest and a copy of the document comes out from the side of the machine.
How are the copies made?
Xeroxing is a photocopying process, which makes use of a combination of
photoconductivity a electrostatic charges. Light is used to discharge regions of
a charged surface to produce a latent image, which is transferred to paper
using a toner. At the heart of the photocopier is a drum made out of
photoconductive material that is first charged positively using a corona wire.
When an intense beam of light is moved across the paper placed on the
copier's glass surface, the image of the document is focussed on the charged
drum. Light reflected from white areas of the paper and falling on the
charged drum neutralises the charge in those areas. Dark areas on the
original (such as pictures or text) do not reflect light onto the drum, leaving
regions of positive charges on the drum's surface intact. When negatively
charged, dry, black pigment called toner is spread over the surface of the
drum, the toner particles adhere only to the positive charges that remain,
creating a temporary image of the original, which is transferred to a positively
charged sheet of paper. The paper is then heated and pressed to fuse the
image formed by the toner to the paper's surface. That is why the paper feels
when it comes out the photocopier.
Since the working of photocopiers depends mostly on static charges, high
humidity affects the quality of photocopies produced.
43. Car transmission gears
Whether you ride a bus or a car you must have noticed that after starting, the
speed of the vehicle increases in steps as the driver changes gears. And he
can't change gears randomly; he has to do it in a certain order -- first, second,
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third, and fourth. Some modern cars even go up to the fifth gear. What do
these 'gears' mean and why changing gears is necessary?
The car or bus engine can produce a certain amount maximum power
depending upon the engine capacity, design and other factors. But a running
vehicle does not require the same level of power at all times. It needs the
maximum power when starting from rest or while climbing a gradient. On
level a road, as the speed increases, less and less power is needed to maintain
the speed. Changing gears allow the driver to transmit the required amount
of power to the wheels.
Internal combustion engines used in cars and other vehicles have narrow rpm
ranges where power and torque are at their maximum. For example, an
engine might produce its maximum power at 5,500 rpm. The transmission
allows the gear ratio between the engine and the drive wheels to change as
the car speeds up and slows down, while maintaining the engine speed at
5,500 rpm. To de-link the driving shaft from the engine during gear change
the transmission is connected to the engine through the clutch.
The transmission allows the 'gear ratio' between the engine and the drive
wheels to change as the car speeds up and slows down, and at the same time
to maintain an optimum power level. Gear ratio refers to the ratio between
number of teeth of two meshing gears, which also decides the ratio of the
speed or rotation of the two gears. In first gear the driving shaft rotates much
slower than the engine shaft, which produces more torque at the drive wheel
required for starting the vehicle from rest. As the vehicle speed increases, less
and less torque is required to maintain the motion and change to higher gears
helps increase the speed, as the gear ratio allows the drive shaft to spin almost
at the rpm of the engine. In cars with automatic transmission the gear ratio is
changed continuously as required.
44. Physics of hammers
We all use a hammer to fix nails or break stone or bricks, or shape metal
sheets. A hammer is basically a tool meant to deliver blows to a target,
causing it to move or deform. Scientifically, a hammer can be looked upon as
a force amplifier that converts mechanical work into kinetic energy and back.
Early humans used lumps of stone to break stone or animal bones to make
tools. The amount of energy delivered to the target by a hand-held stone is
equivalent to one half the mass of the stone times the square of the stone's
speed at the time of impact. When we use a hammer, the handle, by
increasing the radius of the swing, allows us to maximize the speed of the
hammerhead on each blow. Here what really matters is the length of the
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handle and mass of the head. Longer the handle or heavier the head, more is
the energy delivered to the target.
In the swing that precedes each blow, a certain amount of kinetic energy gets
stored in the hammer's head, depending on its mass and the speed of its
motion. When the hammer strikes, the head transfers its momentum to the
target. To be most effective a hammer has to transfer its momentum fastest,
which is why most hammers have hardened steel heads. Steel headed
hammers are suitable for driving nails into wood or brick wall, or for
breaking a piece of stone, or bend a sheet of metal. For really big projects such
as driving wedges into wood and posts into the ground, a sledgehammer,
with massive head and a long handle, is usually used.
45. Corrugated boards
We buy many things that come packed in boxes, be it a TV, VCR, computer,
fridge, even fruits. And the packing box is invariably made of corrugated
board. If you look carefully, you'll find that the board is actually made of
layers of paper in which the middle layer is fluted or corrugated. How does
simple corrugation make paper so stiff and strong to be used as a rigid
packaging material?
If you take a piece of corrugated board apart you'll find that it is a composite
structure made up of three or more layers of paper with different
characteristics. It has two main components: the liner and the medium. Both
are made of a special kind of brown paper. The liner is the flat layer that
adheres to the medium, which in turn is the wavy, fluted paper in between
the liners.
The strength of corrugated board comes from the wavy, fluted middle layer,
which provides reinforcement. If you've made a paper fan you'd know that
folding makes paper more rigid. In corrugated board the folds in the fluted
layer make a series of parallel arches. Architects have known for thousands of
years that an arch with the proper curve is the strongest way to span a given
space. The inventors of corrugated paperboard applied these same principles
to paper when they put arches in the corrugated medium. When anchored to
liners on both sides with an adhesive, they resist bending and pressure from
all directions.
Corrugated board is not only rigid but also has superior cushioning qualities,
which resists crushing under compression and gives cushioning protection to
the box's contents. Containers, boxes and pallets can hold products in an
optimally protective environment, so even heavy or fragile contents can be
transported undamaged.
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46. The violin
The violin is a stringed musical instrument, but unlike many other stringed
instruments like the sitar, sarod, and guitar, which are played by plucking,
the violin is played by bowing. The bow is made of horsehair, pulled taut by a
wooden stick. It is easy to understand how sound is produced in a sitar,
sarod, or guitar; the plucking sets the strings into vibration, which leads to the
production of sound. But how does drawing the bow across the strings make
the violin produce sound?
A bowed string works in an entirely different way. Horsehair of which the
bow is made has a rough surface made of scales. When rosin is applied on the
bow it makes the surface of the hairs sticky, but unevenly so. As the bow hairs
rub across the violin string the sticky areas grab the string and push it
forward a little bit till the string's restoring force overpowers the friction and
string starts sliding backwards in a jerk till grabbed by the next sticky area on
the hairs, and the process goes on as long as the bow is drawn across the
string. As a result, the string vibrates as a harmonic oscillator, with a
frequency depending on the position of the finger of the player on the string.
Apart from difference in the mode of sound production, the violin also differs
from the plucked string instruments in the quality of sound produced. In fact,
every musical instrument has a very separate, distinct sound, called "timbre"
in music lingo, which helps us distinguish between them. Each instrument
has a specific pattern of harmonics, which create the unique sound. A given
note on a violin usually has several frequencies vibrating at once. This distinct
combination creates the uniquely beautiful timbre of the violin.
49. Pole vaulting
Pole-vaulting is an exciting athletic event in which the vaulter, running with a
long pole in hand, jumps over a crossbar placed several metres above ground.
It is a wonderful illustration of how one type of energy is converted to
another type of energy. Through a proper use of the pole, the energy of
motion of the athlete is converted into the energy needed to overcome gravity
and reach a certain height. How does it work?
The crucial component of pole vaulting is the vaulting pole, which is a very
advanced piece of equipment. It is constructed from carbon fibre and
fibreglass composite materials in several layers and is usually 5.00-5.20 m
long. An ideal pole should absorb all of the vaulter's energy while bending,
and then return all of that energy as it straightens out. When the vaulter
reaches the end of his/her run-up and engages the pole in the take-off box,
the pole begins to bend under the effect of the momentum of the vaulter, and
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33. Physics at Work and Play 33
the vaulter and pole system rotates about the take-off box. At this point the
initial kinetic energy of the run-up is transformed into potential energy of the
vaulter above the ground. As the pole bends and recoils, the vaulter rotates
about the shoulders, and then pulls up on the pole so as to pass over the
crossbar feet-first.
What a pole-vaulter would ideally want to achieve is to convert all of his/her
kinetic energy into gravitational potential energy. In the real world, however,
a 100 percent conversion is never possible because some of the kinetic energy
gets converted into other kinds of energy, such as heat, friction, sound, and
vibrations of the pole itself. Nevertheless, pole-vaulters are able to achieve
dizzy heights and today, the world-record is over 6.15 metres.
48. Movie magic
When you go to a movie what you see on the screen appears to move, talk,
dance, as if in real life. But in reality, it is an illusion; the images do not
actually move on the screen. Motion-picture film is a strip of discrete, still
pictures but produces the visual impression of continuous movement. What
you really see on the screen is a sequence of rapidly changing still images
projected on the screen that appear to move because of something happening
in your brain. Your eye and brain retain a visual impression for about 1/30th
of a second. (The exact time depends on the brightness of the image.). So if
two images are shown in quick succession within a period less than 1/30th of
a second the eye won't be able to tell them as separate but will perceive them
as a single merged image. If there is a succession of images showing different
stages of movement of, say, a person, the eye will perceive them as
continuous movement. And that is what happens when you see a movie.
In reality it is a bit more complicated. Early experiments with movies showed
that a minimum of about 10 separate frames must be projected every second
to give the illusion of movement. But this is not enough; the image will flicker
very badly if a projector with only a single shutter (to block the light when the
film moves) is used. Further experiments showed that the flicker rate must be
of the order of 50 per second for it not to be obvious. By the time 'talkies'
arrived a standard speed of 24 frames per second was decided upon, which
meant that a shutter with two blades could be used to project 48 screen
images (each frame shown twice) per second, giving an impression of flicker-
free movement.
Television images are produced at the rate of 30 frames per second, but that
would produce flicker. The problem was solved by using an innovation called
'interlaced scanning', in which each picture or frame is scanned as two half-
frames or fields, each field containing every other line of the frame. This, in
effect, produces flicker-free image at the rate of 60 frames per second.
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34. Physics at Work and Play 34
49. Halogen lamps
Lighting is an essential component of modern living. A wide variety of
lighting types are used to meet specific requirements. At home and in offices
tungsten bulbs, fluorescent tubes, or compact fluorescent lamps are used. For
street lighting sodium lamps have become the norm while mercury vapour
lamps find use in factory floors. But nothing can beat the halogen lamp in
term of the illumination it can give per watt of energy consumed. Halogen
lamps are now routinely used in floodlighting of stadiums, monuments and
buildings, and also in stage lighting.
A normal tungsten bulb is made up of a fairly large, thin, transparent or
frosted glass envelope filled with a gas such as nitrogen, or a mixture of argon
and nitrogen. At the centre of the lamp is a tungsten filament. When current is
passed the filament heats up to about 2,500 degrees Celsius, giving off white
visible light in a process called incandescence. A normal tungsten bulb is not
very efficient because, in the process of radiating light, it also radiates a huge
amount of heat -- far more heat than light. Also at high temperature the
tungsten in the filament evaporates and deposits on the glass. Eventually, the
filament becomes so thin at spots that it breaks, and the bulb "burns out."
A halogen lamp is really a specialized type of incandescent lamp, but it is
much more efficient and gives off brighter light for a longer period. In
halogen lamp the tungsten filament is encased inside a much smaller quartz
envelope, which can withstand much higher temperature than glass. The gas
inside the envelope is also different – it consists of vapours of bromine or
iodine, which belong to the halogen group. Halogen lamps produce whiter
light than normal filament bulbs because they operate at higher temperatures.
If the temperature is high enough, the halogen gas combines with tungsten
atoms as they evaporate and redeposit them on the filament. This recycling
process not only prevents formation of tungsten deposit on the glass but also
lets the filament last a lot longer. Besides, since the filament emits light at a
higher temperature, more light is given off per unit of energy.
50. Carrying load on shoulders
The practice of carrying loads suspended from the opposite ends of a long
pole on shoulder is quite common in India. Snake charmers, milkmen,
vegetable sellers, and many other small traders use the same method to carry
their ware for door-to-door sale or to the market. Kanwarias also carry water in
pots in the same way. But if you look carefully, you'll find that the pole is not
rigid; it's quite flexible and is usually a long piece of split bamboo. As the
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35. Physics at Work and Play 35
person carrying the load walks or runs the load and the pole oscillates
vigorously up and down. Does using a flexible pole really help in any way?
Whenever we walk our shoulder moves up and down in a rhythmic manner.
If we carry load suspended from the ends of a rigid pole on our shoulder then
during the upward motion of the body the shoulder has to apply a large force
to lift the pole and its load. During the downward motion of the shoulder,
however, the pole and its load simply rest on the shoulder. So there is a large
fluctuation in the force the shoulder has to bear as we walk or run carrying
the load, which makes it very tiring.
But things change drastically if we use a flexible pole. If we walk or run with
loads suspended at the ends of a flexible pole, the loads oscillate up and
down, and as a result the force on the shoulder is smoothed out. This is
because, when the loads at the ends move down the centre of the pole moves
up and vice versa. The centre of pole also oscillates out of step with the
shoulder – when the shoulder moves upward the centre of the pole moves
downward and vice versa. The net result is that the shoulder experiences a
nearly constant force, which makes carrying the load less tiring.
*****
Biman Basu