A push or pull is called a force. The SI unit of
force is the Newton(N).
A force acts in a particular direction and may
have any of the following effects on an object:
Make a stationary object start moving or
increase the speed of a object.
Slow down or stop a moving object.
Change the direction of a moving object.
Change the shape of an object.
Some forces are small while others are larger.
Forces therefore have size(magnitude).
A force is represented by a line with an arrow
showing the direction in which it acts ,thus:
TYPES OF FORCES
Cohesive and Adhesive forces
This is the force of attraction between two
bodies of given masses(m1 and m2 ),see figure.
When objects are thrown up from or bear the
earth’s surface, they always fall downwards
towards the groun.This is because of the force of
attraction which the earth exerts on any body
near its surface.
This force which pulls the body towards the
centre of the earth is called the gravitational
force of the earth.
The pull of gravity on the body towards the
centre of the earth is called weight.the weight
of an object varies on diffrent planets because
planets have different gravitational pull.
The pull or compression of a string at both of
its ends is called its tension.
Tension is as a result of two opposing forces
applied,one at each end of a body,see figure 3.3
EXP3.1:to study the relationship between mass
and extension of a spring.
The length of a spring increases when loading
since the weight of the load acts on the
spring,forcing it to strectch.
The graph of extension against mass is a
straight line, see figure 3.5.
Some forces compress bodies and are called
Materials that can be compressed or extended
without breaking are called elastic materials.
There is always an upward force acting on an
object immersed in a fluid(liquid or gas).This
upward force is called upthrust.an object in a
vacuum will not experience upthrust.
If the medium is air,the reading is larger than
when the medium is a liquid.The reading also
varies from one liquid to another.
The reading of the balance is highest when the
object is in air and lowest when the object is
completely submerged in water.
The difference between the readings when the
object is immersed in liquid and when the object
is in air is due to upthrust force.
Cohesive and Adhesive Forces
The forces of attraction between molecules of
the same kind is known as cohesive
force,e.g,between a water molecule and another
water molecule .
while the forces of attraction between
molecules of different kinds is called adhesive
force,e.g.,between water molecules and glass
EXP.3.3:to investigate the behaviour of water on
Water on the clean glass slide spreads on the
glass surface.However ,small drops of water
collect into small spherical balls on the waxed
If mercury is used,small mercury drops in a
clean glass dish surface collect into spherical
balls.Larger mercury drops form oval balls, as in
Water wets the glass surface because the
adhesive forces between the water molecules
and the glass molecules are greater than the
cohesive forces between water molecules.
Water does not wet the waxed glass surface
because the cohesive force is greater than the
The stronger cohesive forces in mercury form
spherical drops of mercury even on clean glass
surface.The weak adhesive force between
mercury and glass makes mercury not wet the
EXP3.4:to demonstrate cohesive and adhesive
forces of liquids on narrow tubes
Apparatus ; narrow tubes with different size
*the level of the water inside the tubes is
higher than that outside the tubes.
Different liquids rise by different
heights,depending on the diameter of the glass
The level of mercury in the tube with a smaller
bore will be lower than in the tube with a larger
Since the adhesive force between the water
and glass molecules is greater than cohesive
force between water molecules,the water rises
up the tube so that more water molecules can be
in contact with the glass.
The force of cohesion within the mercury is
greater than the force of adhesion between
mercury and glass.the mercury sinks downthe
tube to enable mercury molecules to keep
Friction is a force that opposes relative motion
between two surfaces in contact.
Friction is caused by the interlocking of the
surfaces and attractive force between the
Frictional force can be reduced by using
rollers,oiling or smoothening.
As it moves through liquid,it rubs against the
liquid ,molecules.Due to the movement of the
body ,there is opposing force between the layer
of the liquid molecules on the body and the layer
of the liquid molecules in the measuring
cylinder.the opposing force (frictional force)
involving a fluid is called viscous drag
The force which causes attraction or repulsion
by magnet is called magnetic force.
A magnet has two types of poles, anorth and a
South pole.like poles repel while unlike poles
Which get attracted are called magnetic
materials while not attracted are called non-
The rubbing creates static changes .The force
of attraction or repulsion due to these charges is
called electrostatic force.
When glass window is wiped with a dry cloth
on a dry day, dust particles are attracted on it.
Centripental force is a force which contrains a
body to move in acircular path or orbit.This force
is directed towards the centre of the orbit.
Q:Give a reason why the weight of body varies
on different planets.
A;weight depends on gravity-the force of
gravity varies from one place to another.
Q:what do you understand by upthrust force?
A:upthrust is the upward force exerted by a
liquid on an object placed in it.
It is commonly observed that liquids form
drops,water wets some surfaces but runs off
others,some insects like pond skater manage to
rest on the surface of water without
sinking,water rises up a narrow glass tube but
mercury is pushed down to alower level in the
same tube and stell or razor blade floats on
water,even though steel is denser than water.
Floating a needle
If carefully placed on the surface, a small
needle can be made to float on the surface of
water even though it is several times as dense
as water. If the surface is agitated to break up
the surface tension, then needle will quickly
The force which causes the surface of aliquid to
behave like a stretched elastic skin is called
surface tension.this force is due to the force of
attraction between individual molecules of the
Molecular Explanation of Surface Tension
A molecule ,say C deep in the liquid is
surrounded by molecules on all sides so that the
net force on it is zero.However,molecules of the
surface, say A and B will have fewer molecules
on the vapour side and hence will experience a
resultant inward force,causing the surface of the
liquid to be in tension.
To study the behaviour of soap bubbles
Observation :the bubble flattens to a film and
the film slowly rises up the funnel.
Explanation: the soap bubble behaves as if its
surface is tightly stretched.As it tries to make its
surface as small as possible ,it rises up the funnel.
To study the behaviour of soap films
When the film is broken on one side,the thread
assumes a perfect curve.
The thread lies in any position in the soap film
because the thread is being pulled on both sides
by equal forces of surface tension.
When one side of the thread is broken ,the
surface tension acts only on ane sode of thread.
As the water tries to make its surface as small
as possible,it pulls the thread in such a way that
it forms a perfect curve.The soap film exhibits
Examples of Surface Tension
Washing with cold water
The major reason for using hot water for
washing is that its surface tension is lower and it
is a better wetting agent. But if the detergent
lowers the surface tension, the heating may be
Don't touch the tent!
Common tent materials are somewhat
rainproof in that the surface tension of water will
bridge the pores in the finely woven material.
But if you touch the tent material with your
finger, you break the surface tension and the
rain will drip through.
Soaps and detergents
help the cleaning of clothes by lowering the
surface tension of the water so that it more
readily soaks into pores and soiled areas.
Walking on water
Small insects such as the water strider can
walk on water because their weight is not
enough to penetrate the surface.
When a brush is in water ,the bristles spread
but when it is taken out,they cling together,see
Surface Tension of Different Liquids
The surface tension of soap is less than that of
A match stick or small toy boat rubbed at one
end with soap and placed on the surface of
water starts moving immediately.It moves in one
direction only and in such a way that the end
that is not rubbed with soap is always in
front.Any attempt to make it move in the
opposite direction fail,see figure 3.22(a).
The soap at the end of the stick immediately
dissolves in water,therby lowering the surface
tension at that end of the stick .the surface
tension at the other end which is now greater
pulls the stick and makes it move in that
direction.the movement gradually weakens and
ultimately ceases.This happens when the whole
surface of water is covered with soap solution.
Factors Affecting Surface Tension
Impurities reduce the surface tension of a
liquid.Detergents,weaken the cohesive force
between liquid molecules.
With rise in temperature ,the kinetic energy of
the molecules of a liquid is increased.The
intermolecular distance increase and the force of
cohesion is decreased.So,the surface tension is
Consequences of surface Tension
• Water insects can rest on the surface of water
without breaking the surface.
• Mosquito larvae float on water surface
Action and Reaction
Forces are interactions between objects.
A pushes/pulls B --- B pushes/pulls A.
One of these forces is called the action force,
and the other one is called the reaction force - it
doesn't matter which is which. For example, if:
You push book. (Action force)
Book pushes you. (Reaction force)
Action and reaction are equal and opposite,
i.e,when one force acts on a body, an equal and
opposite force acts on another body.
MASS AND WEIGHT
While mass is the quantity of matter present in
an object,weight is a measure of the pull of
gravity on the object.This pull of gravity is
always directed towards the centre of the
earth.So, weight has both direction and size.
The SI unit of weight is the Newton(N).Weight
is measured by a spring balance calibrated in
The weight of an object varies from place to
place .So,a body weighs more at the poles than
Relationship between Mass and Weight
Exp :3.12:To determine the gravitational
field strength `g` of a place
Six 20 g masses ,spring balance calibrated in
newtons ,retort stand.
The value of the slope is called the
gravitational field strength(intensity).This is the
gravitational force on a unit mass at that place
on the earth.
On the earth`s surface, an object of mass m
has a gravitational pull of mg on it,where g is
acceleration due to gravity(free fall).
Thus ,the weight W=mg
The earth pulls each kilogram of mass on its
surface with a force of about 9.8 N,which is
approximately 10 Nkg-1
Calculate the weight of each of the following:
A cat of mass 2.5 kg
A pencil of mass 10 g
A lorry of 8 tonnes.(use g=10N/kg)
A spiral spring extends when a stretching force
is applied.The larger the force,the more the
spring extends.The extension of a spring can
therefore be used to measure an applied force.
Such an instrument that uses the extension of
a spring to measure forces is called a spring
balance,see figure 3.24.
SCALAR AND VECTOR QUANTITIES
A scalar quantity is one that has magnitude
only but no direction.
Vector quantities have both magnitude and
A vector quantity is represented by a straight
line with an arrow –the length of the arrow
represents the magnitude of the vector.
Vectors can be added or subtracted.
The sum of the two or more vectors is called
the resultant vector.