Grade 8 Science IMs Q1 S1.ppt

I. Force
A. Definition. – a push or pull that
causes change in the motion of an
object.
B. Measured in Newton (N) – by a
spring scale
1 Newton = 1 kg.m/s²

C. Forces in combination
1. same direction 50N
50N = 100N
2. opposite direction 100N 
25N=75N

D. Friction – force that slows or prevents
motion
1. Sources
a. roughness of
surface; ex. road, floor

2. Types
a. static friction – friction at rest;
ex. eraser sits still
b. sliding friction – something
pushed across a surface; ex. box
pushed on floor

c. rolling friction – between wheels &
floor; ex. car
d. fluid friction – friction of liquids or
gases; ex. airplane, boat

3. reducing friction
a. lubricants – oil, wax, grease
b. switch from sliding to rolling
c. smooth surface – ex. use
sandpaper

4. increasing friction –
make surfaces rougher
& increase the weight
(forces pushing the
surfaces together

Why did the teacher insist that her
students wear rain slickers?
She wanted to reduce the friction
between them.

E. Gravitational Force – force of
attraction between any 2 objects
that have mass (Newton)
1. Law of universal
gravitation – all matter
experiences gravity; the size
of the force depends on the
masses of the objects & the
distance between them

a. size:
sunearthmoon
(tides)
b. distance – earth’s
gravity affects us more

force which pulls
objects toward the
center of a curving
path

II. Motion – occurs when an object
changes position over time
A. Types of Motion
1. neither direction
nor speed changes

4. opposite forces
5. vertical

B. Motion described
1. relative position to a reference point; ex.
moving past the middle school, comet
moved past the sun
2. Energy- Law of conservation of energy:
energy is neither created nor destroyed;
a. potential – stored energy; ex. car @ top of
hill
b. kinetic – energy in motion; ex. car moving

3. direction – N, S, E, W
4. speed – rate at which
object moves over time

a.) speed = distance/time
ex. 100 miles/2 hrs. =
50 miles/hr
b.) time = distance/speed
ex. 100 miles/ 50

What is meant by
unbalanced force?
If the forces on an object are equal and
opposite, they are said to be balanced, and the
object experiences no change in motion. If
they are not equal and opposite, then the
forces are unbalanced and the motion of the
object changes.

Some Examples from Real Life
Two teams are playing tug of war. They are
both exerting equal force on the rope in
opposite directions. This balanced force results
in no change of motion.
A soccer ball is sitting at rest. It
takes an unbalanced force of a kick
to change its motion.

More Examples from Real Life
A powerful locomotive begins to pull a
long line of boxcars that were sitting at
rest. Since the boxcars are so massive,
they have a great deal of inertia and it
takes a large force to change their
motion. Once they are moving, it takes
a large force to stop them.
On your way to school, a bug
flies into your windshield. Since
the bug is so small, it has very
little inertia and exerts a very
small force on your car (so small
that you don’t even feel it).

If objects in motion tend to stay in motion, why don’t
moving objects keep moving forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon it.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.

In outer space, away from gravity and any
sources of friction, a rocket ship launched
with a certain speed and direction would
keep going in that same direction and at that
same speed forever.

More about F = ma
If you double the mass, you double the force. If you
double the acceleration, you double the force.
What if you double the mass and the acceleration?
(2m)(2a) = 4F
Doubling the mass and the acceleration quadruples
the force.
So . . . what if you decrease the mass by half? How
much force would the object have now?

What does F = ma
say?
F = ma basically means that the force of an object
comes from its mass and its acceleration.
Something very small (low mass) that’s
changing speed very quickly (high
acceleration), like a bullet, can still
have a great force. Something very
small changing speed very slowly will
have a very weak force.
Something very massive (high mass)
that’s changing speed very slowly (low
acceleration), like a glacier, can still
have great force.

Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall? You slide in the opposite direction
(away from the wall), because you pushed on
the wall but the wall pushed back on you with
equal and opposite force.
Why does it hurt so much when you stub
your toe? When your toe exerts a force on a
rock, the rock exerts an equal force back on
your toe. The harder you hit your toe against
it, the more force the rock exerts back on your
toe (and the more your toe hurts).

Newton’s Contributions
Calculus
Light is composed of
rainbow colors
Reflecting Telescope
Laws of Motion
Theory of
Gravitation

Newton’s
Laws of Motion
I. The Law of Inertia
II. The Law of Acceleration (F=ma)
III. The Law of Interaction (Action-
Reaction)

While most people know what
Newton's laws say, many people do
not know what they mean (or simply
do not believe what they mean).

Newton’s Laws of Motion
1st Law – An object at
rest will stay at rest,
and an object in
motion will stay in
motion at constant
velocity, unless acted

1st Law of Motion
(Law of Inertia)
An object at rest will stay
at rest, and an object in
motion will stay in
motion at constant
velocity, unless acted
upon by an unbalanced

Balanced Force
Equal forces in opposite
directions produce no motion

Unbalanced Forces
Unequal opposing forces
produce an unbalanced force
causing motion

If objects in motion tend to stay in motion, why don’t
moving objects keep moving forever?
Things don’t keep moving forever because
there’s almost always an unbalanced force
acting upon them.
A book sliding across a table slows
down and stops because of the force
of friction.
If you throw a ball upwards it will
eventually slow down and fall
because of the force of gravity.

Newton’s First Law
(law of inertia)
MASS is the measure of the amount of
matter in an object.
It is measured in Kilograms

1st Law
Inertia is the
tendency of
an object to
resist
changes in
its velocity:
whether in
motion or
These pumpkins will not move unless acted on
by an unbalanced force.

Newton’s First Law
(law of inertia)
INERTIA is a property of an object that
describes how ______________________ the
motion of the object
more _____ means more ____
much it will resist change to
mass inertia

1st Law
Once
airborne,
unless acted
on by an
unbalanced
force (gravity
and air – fluid
friction), it
would never

1st Law
Unless acted upon
by an unbalanced
force, this golf ball
would sit on the tee
forever.

What is this
unbalanced force
that acts on an
object in motion?

There are four main
types of friction:
Sliding friction: ice skating
Rolling friction: bowling
Fluid friction (air or liquid): air or water resistance
What is this unbalanced force that acts on an
object in motion?

Why then, do we observe every day
objects in motion slowing down and
becoming motionless seemingly without
an outside force?
It’s a force we sometimes cannot see – friction.

Objects on earth, unlike the
frictionless space the moon travels
through, are under the influence of
friction.

Slide a book across a
table and watch it slide to
a rest position. The book
comes to a rest because of
the presence of a force -
that force being the force
of friction - which brings
the book to a rest position.

In the absence of a force of
friction, the book would continue
in motion with the same speed
and direction - forever! (Or at
least to the end of the table top.)

Newtons’s 1st Law and You
Don’t let this be you. Wear seat belts.
Because of inertia, objects (including you)
resist changes in their motion. When the car
going 80 km/hour is stopped by the brick wall,
your body keeps moving at 80 m/hour.

Newton’s Second Law-
LAW OF ACCELERATION

2nd Law
The net force of an object is
equal to the product of its mass
and acceleration, or F=ma.
Law of Acceleration- states that
acceleration is directly
proportional to the magnitude of a
net force, and follows the
direction of that net force, while it
is inversely proportional to the

2nd Law
•When mass is in kilograms and
acceleration is in m/s/s, the unit
of force is in newtons (N).
•One newton is equal to the force
required to accelerate one

Newton’s Second Law
Force = Mass x
Acceleration
Force is measured in
Newtons
ACCELERATION of GRAVITY(Earth) = 9.8 m/s2
Weight (force) = mass x
gravity (Earth)
Moon’s gravity is 1/6 of the Earth’s
If you weigh 420 Newton on earth, what will you weigh
on the Moon?
70Newton
If your mass is 41.5Kg on Earth what is your mass
on the Moon?

One rock weighs 5 Newtons.
The other rock weighs 0.5
Newtons. How much more
force will be required to
accelerate the first rock
at the same rate as the
second rock?
Ten times as much

WEIGHT is a measure of the force of
________ on the mass of an object
measured in _________
gravity
Newton

2nd Law (F = m x a)
How much force is needed to
accelerate a 1400 kilogram car 2
meters per second/per second?
Write the formula
F = m x a
Fill in given numbers and units

Newton’s 2nd Law proves that different masses
accelerate to the earth at the same rate, but with
different forces.
• We know that objects
with different masses
accelerate to the
ground at the same
rate.
• However, because of
the 2nd Law we know
that they don’t hit the
ground with the same
force.
F = ma
98 N = 10 kg x 9.8 m/s/s
F = ma
9.8 N = 1 kg x 9.8 m/s/s

Check Your
Understanding
1. What acceleration
will result when a 12 N
net force applied to a 3
kg object? A 6 kg
object?
2. A net force of 16 N
causes a mass to

Check Your Understanding
• 1. What acceleration will result when a 12 N net force applied to a 3 kg object?
12 N = 3 kg x 4 m/s/s
• 2. A net force of 16 N causes a mass to accelerate at a rate of 5 m/s2. Determine the mass.
16 N = 3.2 kg x 5 m/s/s
• 3. How much force is needed to accelerate a 66 kg skier 1 m/sec/sec?
66 kg-m/sec/sec or 66 N
• 4. What is the force on a 1000 kg elevator that is falling freely at 9.8 m/sec/sec?
• 9800 kg-m/sec/sec or 9800 N

3rd Law
For every action, there is an equal
and opposite reaction.
LAW OF
INTERACTION

Newton’s 3rd Law
For every action there
is an equal and
opposite reaction.
Book to
earth
Table to
book

3rd Law
According to
Newton, whenever
objects A and B
interact with each
other, they exert
forces upon each
other. When you sit
in your chair, your
body exerts a
downward force on
the chair and the
chair exerts an

3rd Law
There are two forces
resulting from this
interaction - a force
on the chair and a
force on your body.
These two forces
are called action
and reaction forces.

Think about it . . .
What happens if you are standing on a
skateboard or a slippery floor and push against
a wall?
You slide in the opposite direction (away from
the wall), because you pushed on the wall but
the wall pushed back on you with equal and
opposite force.
Why does it hurt so much when you stub
your toe?
When your toe exerts a force on a rock, the
rock exerts an equal force back on your toe.
The harder you hit your toe against it, the
more force the rock exerts back on your toe
(and the more your toe hurts).

Newton’s 3rd Law in Nature
Consider the
propulsion of a fish
through the water. A
fish uses its fins to
push water backwards.
In turn, the water reacts
by pushing the fish
forwards, propelling the
fish through the water.
The size of the force on
the water equals the
size of the force on the
fish; the direction of the
force on the water
(backwards) is opposite
the direction of the

3rd Law
Flying gracefully
through the air, birds
depend on Newton’s
third law of motion. As
the birds push down
on the air with their
wings, the air pushes
their wings up and
gives them lift.

Consider the flying motion of birds. A bird flies by use of its
wings. The wings of a bird push air downwards. In turn, the air
reacts by pushing the bird upwards.
The size of the force on the air equals the size of the force on
the bird; the direction of the force on the air (downwards) is
opposite the direction of the force on the bird (upwards).
Action-reaction force pairs make it possible for birds to fly.

Other examples of Newton’s Third Law
The baseball forces the bat
to the left (an action); the
bat forces the ball to the
right (the reaction).

3rd Law
Consider the motion of a car
on the way to school. A car
is equipped with wheels
which spin backwards. As
the wheels spin backwards,
they grip the road and push
the road backwards.

3rd Law
The reaction of a rocket is
an application of the third
law of motion. Various
fuels are burned in the
engine, producing hot
gases.
The hot gases push against
the inside tube of the rocket
and escape out the bottom
of the tube. As the gases
move downward, the rocket
moves in the opposite
direction.

Newton’s Third Law
A bug with a mass of 5
grams flies into the
windshield of a moving
1000kg bus.
Which will have the most
force?
The bug on the bus
The bus on the bug

Newton’s Third Law
The force would be the
same.
Force (bug)= m x A
Force (bus)= M x a
Think I look bad?
You should see
the other guy!

Action: earth pulls on you
Reaction: you pull on earth
Action and Reaction on Different Masses
Consider you and the earth

Action: tire pushes on road
Reaction: road pushes on tire

Action: rocket pushes on gases
Reaction: gases push on rocket

Consider hitting a baseball with a bat. If
we call the force applied to the ball by the
bat the action force, identify the reaction
force.
(a) the force applied to
the bat by the hands
(b) the force applied to
the bat by the ball
(b) the force applied to the bat by the ball

Newton’s 3rd Law
Suppose you are taking a space walk near
the space shuttle, and your safety line
breaks. How would you get back to the
shuttle?

Newton’s 3rd Law
The thing to do would be to take one of the tools from your tool
belt and throw it is hard as you can directly away from the
shuttle. Then, with the help of Newton's second and third laws,
you will accelerate back towards the shuttle. As you throw the
tool, you push against it, causing it to accelerate. At the same
time, by Newton's third law, the tool is pushing back against
you in the opposite direction, which causes you to accelerate
back towards the shuttle, as desired.

Review
Newton’s First Law:
Objects in motion tend to stay in motion
and objects at rest tend to stay at rest
unless acted upon by an unbalanced force.
Newton’s Second Law:
Force equals mass times acceleration
(F = ma).
Newton’s Third Law:
For every action there is an equal and
opposite reaction.

1stlaw: Homer is large and
has much mass, therefore he
has much inertia. Friction
and gravity oppose his
motion.
2nd law: Homer’s mass x
9.8 m/s/s equals his
weight, which is a force.
3rd law: Homer pushes
against the ground and it
pushes back.

OBJECTIVES
a. describe properties and characteristics of
light
b. explain the occurrence of spectacular
events in the sky like rainbows, red
sunset/sunrise and blue sky
c. infer that white light is made up of different
colors of light and color black is actually the
absence of all colors

Characteristics and Properties of Sound

A1.The Disappearing
Color Wheel-
page 86

1. What does spinning the color wheel
do to the colors on it?
Answer: Spinning the color wheel
blends the color on it.
2. What color did you see when you
spun the color wheel?
Answer: The color white is seen.

3. Why did the color disappear when you spun
the color wheel?
Answer: The colors disappear because the
colors blended together and made the color
white.
4. Based on the experiment, what can you infer
about the color white?
Answer: The color white is actually the color
of light when all colors are present and mixed
together.

The pictures show properties of light
and how such property is being
processed.
Questions:
1. Can you tell the properties of light
being exhibited?
-reflection, refraction, diffraction,
interference, absorption

Light is both a
wave and a
particle, it has a
dual property

2. How do light travels?
- Light travels as a wave. But unlike sound waves or water
waves, it does not need any matter or material to carry its
energy along. This means that light can travel through a
vacuum—a completely airless space. ... It speeds through
the vacuum of space at approximately 300,000 meter per
second.

SPEED OF LIGHT
One of the most famous experiments
on the speed of light was conducted by
American physicist named ALBERT
MICHELSON on 1880.
He used a light source, a mirror, and a
telescope for his experiment.
Light travel at an extremely fast speed

With the speed of 300 000 m/s, the light
from the sun takes around only eight
minutes to reach Earth.
With this speed, light can make 7.5 rounds
around Earth in just one second!
The distance that light travels in year is
called light-year.
The diameter of a Milky Way is 100 000
light-years, this means it will take around
100 000 years for light to travel across over
enormously galaxy.

3. What kind of wave is light wave?
Electromagnetic waves are made of
oscillating magnetic and electric fields
and, like all waves, they carry energy.
There are many types of electromagnetic
waves.
From lowest energy to highest energy
(red to blue) there are radio waves,
microwaves, infrared, visible light,
ultraviolet, x-rays and gamma rays.

Electromagnetic wave- is a wave
that is partly magnetic and partly
electric.
Visible light is an electromagnetic
wave that makes up the
electromagnetic spectrum

LIGHT AND THE
ELECTROMAGNETIC
SPECTRUM
The movements and acceleration of electrons
in atoms emit the light energy.
It is divided into different categories of waves
that fall within a certain range of frequencies.
Arranged from lowest to highest frequencies (
radio waves, microwaves, infrared, visible
light, ultraviolet, X-rays, and Gamma rays)

Red- lowest frequency
Violet- highest frequency
Infrared waves- come right before the
color red of visible light and they are
emitted when using heat lamps
Ultraviolet waves – emitted by the sun
come after the color violet
- long exposure to ultraviolet rays can
cause sunburn

4. When it bounces off and bends what
property is similar to sound wave?
5. What property of light is common to sound
waves?
- Sound travels as longitudinal waves and needs
to travel through a solid, liquid or gas: it cannot
travel through a vacuum. Light and sound can
be reflected and refracted, just like water waves.
Light and sound can also be diffracted, just
like water waves, but diffraction in light is less
obvious than in sound.

LIGHT AND COLOR
 Isaac Newton tried an experiments wherein he let
sunlight pass through a prism and the sunlight came
through as a spectrum of colors.

Sunlight, which is naturally
white, actually contained all
the colors.

 When you see Black-
no color or light is present

 The color that are neither white nor black
have their specific colors due to the pigment
they have.
PIGMENTS- are substances that absorbed
selected frequencies of light energy and
transmit or reflect the rest.

ABSORPTION
Electrons of atoms vibrate at a specific
frequency as the electrons natural
frequency hits the atom.
This will enable to absorb the light energy
and turn it into thermal energy.
Subsequently, the light wave with that
given frequency is absorbed by the object,
never again to be released in the form of
light.

TRANSMISSION
AND REFLECTION
When frequency of light waves
do not match the natural
frequency of objects that they
are in contact with the object
electrons will only vibrate weakly
for a short period of time.

If the object is
transparent, then the
vibrations of the electrons
are passed on to
neighboring atoms through
the bulk of the material and
reemitted on the opposite
side of the object.

In opaque objects, only the
electrons on the object’s surface
vibrate abruptly, and then they
reemit the light energy as
reflected light
- It can either be a.) absorb all of
the light or b.) reflect it back to the
source

THE BLUE SKY
Sunlight scatters in the atmosphere when
it hits molecules and the larger specks of
matter known as Tyndall effect or
Tyndall scattering
Sunlight radiates waves of visible light
and ultraviolet rays (absorbed by the
ozone layer and the remaining rays
scatter through the atmosphere)

Visible light is scattered as the
following colors from the most to
the least – violet, blue, green,
yellow, orange and red
Eyes is sensitive to the blue light
only because violet light has
frequencies that are too high to
see with the naked eye- reason why
the sky appears blue

THE RED SUNRISE & SUNSET
During sunrise or sunset, sunlight reaches Earth
longer than it does when it shine during noontime.
The visible light travels a longer distance, and is
scattered more because it has to pass through a
thick layer of atmosphere
The color blue in the sky is scattered so much that
it is barely transmitted
The colors with lower frequencies like red,
orange and yellow are scattered less and are
more readily transmitted

THE COLORFUL RAINBOW
A rainbow is simply a group of circular or
nearly circular arcs of color that appear
as a huge arch in the heavens.
The raindrops act like miniature prisms,
refracting or breaking sunlight into various
colors as well as reflecting it to produce
the spectrum.

We can see rainbows in the sky when it is
drizzling from one part of the sky while the
sun is shinning on the opposite side
This colorful arc is caused by sunlight that
was reflected and refracted by the spherical
raindrops
When sunlight hits raindrop, it is first
refracted and dispersed into its spectral
colors
Violet is bent the most and red is bent the
least

A. The first refraction happens when
the visible light coming from the sun
enters one part of the raindrop
B. Some parts of the visible light tends
to reflect back depending on how
bent the colors are
C. The second refraction happens as
the refracted light escapes the
raindrop

rbivaonsit
= VIBRATIONS
mocerpsison
= COMPRESSION
123

1. cafaerrnoti
= RAREFACTION
1. ecislebd
= DECIBELS 124

1.zehrt
= HERTZ
1.ensidyt
= DENSITY 125

1. ytctsleaii
= ELASTICITY
1. rasoinctul
usnods
= ULTRASONIC 126

1. osarfnicin nosusd
= INFRASONIC
SOUNDS
1. Ticph
= PITCH 127

Objectives:
At the end of 60 minutes discussion:
a. I will be able to define what is
sound
b. I will be able to know the science
of how they are produced, how I
perceive them, and how fast they
travel in different mediums, and
c. I will be able to explain how the
properties of different medium s 128

Direction:
In a One Whole paper write T
at the Front of the paper for
The statement about the
underlined word or phrase is
True and F if at the back of the
paper for statement about the
underlined word or phrase is 129

1. Sounds travel
through waves
caused by
vibrations.
2. Compression is one
way that a
longitudinal sound 130

3. Rarefaction is
the bending of
sound waves.
4. Pitch is
measured in
131

5. The human ear can
only hear sounds
between 20 and 20
000 hertz.
6. The loudness of a
sound is
132

7. Infrasonic sounds
are so low that you
cannot hear them.
8. The loudest
sound that you hear
are called ultrasonic
133

9. Elasticity is a
property of matter that
is applicable only to
objects that can
stretch.
10. The amount of
space an object 134

135
Sound
The Nature of Sound
Sounds and the Human Ear

The Look of Sound
Sound Waveforms
Frequency Content
Digital Sampling

137
wavelength
compressed gas
rarefied gas
amplitude
trough
crest
crest
trough

138
Properties of Waves
• Wavelength () is measured from crest-to-crest
– or trough-to-trough, or upswing to upswing, etc.
• For traveling waves (sound, light, water), there is a speed (c)
• Frequency (f) refers to how many cycles pass by per second
– measured in Hertz, or Hz: cycles per second
– associated with this is period: T = 1/f
• These three are closely related:
f = c
 or T
horizontal axis could be:
space: representing
snapshot in time
time: representing
sequence at a par-
ticular point in space
pressure

What IS Sound?
•Sound is really tiny
fluctuations of air pressure
–units of pressure: N/m2 or psi
(lbs/square-inch)
In Physics, Sounds are
waves that carry energy
139

Carried through air at 345
m/s (770 m.p.h) as
compressions and
rarefactions in air pressure
140

back-and-forth motion along
the medium it is traveling
through
141

142
• Speakers vibrate and push on the air
–pushing out creates
compression (is a region in
the sound wave wherein
there is a maximum
pressure)
–pulling back creates
rarefaction (is the region in
the sound wave where there
is minimum pressure)

•What are the sounds that
we can hear?
•How can we hear
sounds?
143

144
Sound Waves
Sound waves travel
as compression
waves.
Another name for
compression waves
is longitudinal waves

The sounds that we
hear are
Longitudinal Waves
that travel through a
medium, and they
are caused by 145

How can we
create Vibrations?
- Clapping your hands,
strumming the guitar,
stomping your feet, or
using machines
146

How are those
following examples
(clapping your hands)
create sound?
- It create a pulse or
disturbance in the air
or on the ground 147

The Frequency of waves
produced is equal to the
Vibrations produced.
The Air and the Ground are
the medium where the
sounds travels through
Sound cannot travel in a
vacuum; it always need
matter to travel 148

Spring 2006
UCSD: Physics 8; 2006
149
Longitudinal vs. Transverse Waves
• Sound is a longitudinal wave, meaning that the motion of particles is
along the direction of propagation
• Transverse waves—water waves, light—have things moving
perpendicular to the direction of propagation

Spring 2006
150
Why is Sound Longitudinal?
• Waves in air can’t really be transverse, because the atoms/molecules
are not bound to each other
– can’t pull a (momentarily) neighboring molecule sideways
– only if a “rubber band” connected the molecules would this work
– fancy way of saying this: gases can’t support shear loads
• Air molecules can really only bump into one another
• Imagine people in a crowded train station with hands in pockets
– pushing into crowd would send a wave of compression into the crowd in the
direction of push (longitudinal)
– jerking people back and forth (sideways, over several meters) would not propagate
into the crowd
– but if everyone held hands (bonds), this transverse motion would propagate into
crowd

151
Factors affecting speed of
sound
Temperature – Sound moves
through hot air faster than cold
air due to the increased speed
of the particles.
Type of medium – Sound
moves well through dense
material and material that
returns quickly to its original

152
Factors affecting speed of
sound
Thus, sound moves fastest in a solid and
slowest in a gas
Sound travels more quickly through solids
and liquids because the individual
molecules are closer together than the
molecules in gas
Sound waves CANNOT move through a
vacuum – they need a medium!

and the
Human Ear
Characteristic
s of Sound 153

amplitude
Related to intensity of
sound
Intensity- is proportional
to the square of the
amplitude of a sound wave
Can be measured with a
tool- oscilloscope 154

Loudness
Is determined by the
amplitude of the
sound waves
Larger
amplitude=more
energetic 155

Human ears can
withstand sound
intensity levels from
zero dB to 120 dB
The threshold of
hearing is at 0 dB,
whereas the threshold
156

Decibel (dB)- unit used
for intensity, base unit
bel (B) named after
Alexander Graham Bell-
inventor of telephone
1 bel= 10 decibels (the
intensity of sound
increases by power of 10)157

Quality
It is used to
distinguished between
two different sounds that
have the same pitch and
loudness
The tone quality depends
158

Frequency
Describes how high or
low a sound is
High pitched sound
(chalk screeching on
board, person singing a
very high note)=their
vibrations have high 159

Low-pitched sound
(big and heavy truck
passing by, a person
singing a very low
note)=their vibrations
have a low frequency 160

Measured in Hertz (Hz)
Infrasonic Sound- are
really low sounds with
frequencies lower than
20Hz
Ultrasonic Sounds- are
really high sounds with
frequencies greater than 161

Human beings can respond
to frequencies from 20 hertz
to 20 000 hertz- AUDIBLE
RANGE
162

How does the
Human Voice Box
Work?
163

Spring 2006
164
Sensitivity of the Human Ear
• We can hear sounds with frequencies ranging from 20 Hz to 20,000 Hz
– an impressive range of three decades (logarithmically)
– about 10 octaves (factors of two)
– compare this to vision, with less than one octave!

Spring 2006
165
Sound hitting your eardrum
• Pressure variations displace membrane (eardrum, microphone) which
can be used to measure sound
– my speaking voice is moving your eardrum by a mere 1.510-4 mm = 150 nm = 1/4
wavelength of visible light!
– threshold of hearing detects 510-8 mm motion, one-half the diameter of a single
atom!!!
– pain threshold corresponds to 0.05 mm displacement
• Ear ignores changes slower than 20 Hz
– so though pressure changes even as you climb stairs, it is too slow to perceive as
sound
• Eardrum can’t be wiggled faster than about 20 kHz
– just like trying to wiggle resonant system too fast produces no significant motion

Speed of Sound
Have you ever wonder
why thunders almost
always come after
lightning?
166

- Sound travels at a
slower speed than light
- Speed of sound is a
millionth (1/1,000,000) of
the speed of light)
- Varies depending on the
properties of medium it
is travelling 167

Spring 2006
168
Speed of Sound
• Sound speed in air is related to the frantic motions of molecules as they
jostle and collide
– since air has a lot of empty space, the communication that a wave is coming
through has to be carried by the motion of particles
– for air, this motion is about 500 m/s, but only about 350 m/s directed in any
particular direction
• Solids have faster sound speeds because atoms are hooked up by
“springs” (bonds)
– don’t have to rely on atoms to traverse gap
– spring compression can (and does) travel faster than actual atom motion

Spring 2006
169
Example Sound Speeds
Medium sound speed (m/s)
air (20C) 343
water 1497
gold 3240
brick 3650
wood 3800–4600
glass 5100
steel 5790
aluminum 6420
http://hypertextbook.com/physics/waves/sound/

Medium - Elasticity
Ability of a material to return
to its original shape after a
certain amount of force is
applied to it
( ex: Steel)
170

Sound travels faster
in more elastic
objects ( Solid- liquid-
gas)
Sound travels fastest
in solids and slowest 171

Imagine if you were to
communicate with someone
who has a hundred meters
away from you. You would have
to shout if you use the AIR as a
medium of the sound. However,
if you allowed the sound to
travel through a solid, such as
the string of improvised phones
made from tin cans, you would
172

Medium- Density
Is an intrinsic property of a
material that is determined by
the amount of mass per area
of space or volume of the
material
173

Denser objects have more
mass per area, and they have
more compact atoms and
molecules
174

177
Decibels
The decibel (dB) is the unit used
to measure sound intensity or
loudness.
Loudness corresponds to the
amplitude of a wave.

178
Decibels
Sounds greater than 120 dB can cause
pain in human ears.
Sounds with an amplitude between 0
dB and 120 dB are called audible.
Anything below 0 dB is considered
subaudible.

179
Frequency and Pitch
Pitch – how high or low a sound is.
High pitch = high frequency
Low pitch = low frequency

180
Frequency and Pitch
Humans can hear pitches that have a
frequency between 20 Hz and 20,000
Hz
Pitches above 20,000 Hz are called
ultrasonic.
Pitches below 20 Hz are called
subsonic.

181
Doppler Effect
The Doppler effect is a change in the
frequency or pitch of a sound that is caused
by either the movement of the source or the
observer of the wave.
Example: The sound from an ambulance
siren changes as it approaches the listener

182
Doppler Effect
First observed in 1842 by
Christian Andreas Doppler
The Big Bang Theory - Doppler Costume
Video explaining the Physics of the Doppler Effect

183
Resonance
Resonance is the vibration of an object
at its natural frequency.
Example: Windows rattle when the
sound from a passing truck matches the
window’s natural resonance.

184
Nodes and Anti-nodes
Node – A place
where two waves
meet and
destructively
interfere so that
the displacement
is zero

185
Nodes and Anti-nodes
Anti-node – the
point of largest
amplitude when
two waves
interfere
constructively

186
Music – Natural Frequency
A natural frequency exists
without any driving source.
It is a natural frequency if its
waveform has nodes that match
up with the ends of the object.
The lowest frequency at which
this occurs is the fundamental,
or the 1st Harmonic.

187
Music - Harmonics
Harmonics – a sound wave with a pitch
that is a multiple of the natural frequency
Overtone – has a higher frequency than
the fundamental
Octave = ½ or double the frequency of a
sound; 8 notes on the musical scale

189
Music
Consonance – multiple waves combining to
form a pleasant sound
Dissonance – multiple waves combining to
form an unpleasant sound
Acoustics – the control of noise and the
vibrations that cause noise

191
Music – open pipe resonators
Open pipe resonators – both ends are
open
Examples: brass instruments, flutes,
saxophones
It reflects an inverted wave

192
Music - Closed pipe resonators
Closed pipe resonators – have
one end enclosed
Example: pan-flute, blowing across
a bottle top, hanging pipes under
marimbas, xylophones

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