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17.1 Mechanical Waves
Pearson Prentice Hall
Physical Science: Concepts in Action
Objectives:
Chapter 17
Mechanical Waves
1. Explain
what causes
mechanical waves
2. Name and describe the
three main types of
mechanical waves
The Nature of Waves
Bellringer

What is your definition of a wave?
What’s in a wave?
• A wave is a repeating disturbance
or movement that transfers energy
through matter or space.
•
The Nature of Waves
Examples?
The Nature of Waves
11.1
Waves and Energy
• Because it is moving, the falling
pebble has energy.
• As it splashes into the pool, the
pebble transfers some of its
energy to nearby water molecules,
causing them to move.
• What you see is energy traveling
in the form of a wave on the
surface of the water.
Waves and Matter
• Imagine you’re in a boat on a lake.
• Approaching
waves bump
against your
boat, but they
don’t carry it
along with
them as they
pass.
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The Nature of Waves
The Nature of Waves
Waves and Matter
• The waves don’t even carry the
water along with them. Only the
energy carried by the waves
moves forward.
• A wave will travel only as long as
it has energy to carry.
• All waves have this propertythey
carry energy without transporting
matter from place to place.
The Nature of Waves
The Nature of Waves
11.
Making Waves
Making Waves
• It is the up-and-down motion of
your hand that creates the wave.
• Anything that moves up and down
or back and forth in a rhythmic way
is vibrating.
• The
vibrating movement of your
hand at the end of the rope
created the wave. In fact,
-All waves are produced by something that vibrates.
What Causes Mechanical Waves
 Def: A
wave is a disturbance that carries
energy through space or matter
 Def: What a wave travels through is the
medium (sound-air, earthquake-ground)
 Def: Waves that require a medium to travel
are called mechanical waves (majority of
waves)
 Mechanical waves carry energy from one
place to another by using matter (a medium)
 Mechanical waves are started with a vibration
The Nature of Waves
Mechanical Waves
• Medium- matter the waves travel
through.
• The medium can be a solid, a
liquid, or gas
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Types of Mechanical Waves
•
The three main types of mechanical waves
are transverse waves, (compressional)
longitudinal waves and surface waves
The Nature of Waves
11.1
Mechanical Waves
• mechanical waves- Waves that can travel
only through a medium.
• Not all waves need a medium.
• Light and radio waves, can travel through
space. (No Medium)
•Particles in a medium can vibrate up and
down or back and forth as a wave moves by
•Def: If the particles move up and down they
will move perpendicular to the direction of
the wave-called a transverse wave
Transverse waves take the shape of
sine curves (looks like an s on its side)
• Def: High points are crests
• Def: Low points are troughs
• Def: Difference between high and low
is called amplitude
• Bigger amplitudes mean more energy
• Def: in a longitudinal wave the medium
moves in the same direction as the
wave
•
The Nature of Waves
Transverse Waves
• transverse wave, Waves move at
right angles to the direction that the
wave travels. (Like water waves)
• For example, a
water wave
travels
horizontally as
the water
moves
vertically up
and down.
The Nature of Waves
Bellringer

What is a transverse wave?
Compressional Waves
• compressional wave, waves move
the same direction that the wave
travels.
• You can model
compressional
waves with a
coiled spring
toy.
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The Nature of Waves
Compressional Waves
• Squeeze several coils together at
one end of the spring. Then let go
of the coils.
•
•
•
•
•
•
Def: In longitudinal waves the bunched area is a
compression (think of a slinky)
Def: The spaced out areas are called rarefactions
Def: Amplitude on a longitudinal wave is
maximum deviation from normal density or
pressure
At the boundary between two mediums (on the
ocean for example) surface waves develop
Def: Surface waves are combinations of both
types of waves
The particles in the medium of a surface wave
move back and forth and up and down resulting
in a circle, but they end up where they started
The Nature of Waves
Sound Waves
The Nature of Waves
Compressional Waves
• As the wave moves, it looks as if
the whole spring is moving toward
• one
The end.
wave carries energy, but not
matter, forward along the spring.
• Compressional
waves also are
called
longitudinal
waves.
The Nature of Waves
Sound Waves
• Sound waves are compressional
waves.
• When a noise is made, such as
when a locker door slams shut
and vibrates, nearby air
molecules are pushed together
by the vibrations.
The Nature of Waves
Sound in Other Materials
• The air molecules are squeezed
together like the coils in a coiled
spring toy are when you make a
compressional wave with it.
• Sound waves also can travel
through other mediums, such as
water and wood.
• The compressions travel through
the air to make a wave.
• When a sound wave reaches your
ear, it causes your eardrum to
vibrate.
• Your inner ear then sends signals
to your brain, and your brain
interprets the signals as sound.
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The Nature of Waves
Water Waves
• Ocean waves are formed most often by
wind blowing across the ocean surface.
• The size of the
waves that are
formed depend on
the wind speed, the
distance over which
the wind blows, and
how long the wind
blows.
The Nature of Waves
Seismic Waves
• Forces in Earth’s crust can cause
regions of the crust to shift, bend,
or even break.
• The breaking
crust vibrates,
creating
seismic (SIZE
mihk) waves
that carry
energy
outward.
The Nature of Waves
Seismic Waves
• Seismic waves are a combination
of compressional and transverse
waves. They can travel through
Earth and along Earth’s surface.
• The more the
crust moves
during an
earthquake, the
more energy is
released.
Evidence of a Surface Transverse Wave
Click image to view movie
Section Check
Question 1
What is a wave?
Answer
A wave is a repeating movement
that transfers energy through
matter or space.
Section Check
Question 2
Which is carried by a water wave?
A.
B.
C.
D.
a boat on the surface
boat anchor submerged 50 m
energy
water molecules
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Section Check
Answer
The answer is C. Waves carry
energy without transporting matter
from place to place.
Section Check
Question 3
Which type of wave does not need a
medium?
A.
B.
C.
D.
electromagnetic
mechanical
ocean
sound
Section Check
Bellringer
Answer
The answer is A. Electromagnetic
waves are made by vibrating
electric charges and can travel
through space where matter is not
present.
17.2 Properties of Mechanical Waves
 Objectives:
 1. Explain
what determines the
frequency of a wave
 2. Solve problems for frequency,
wavelength and speed
 3. Describe how amplitude and
energy are related

What is the difference between a
transverse and a
compressional/longitudinal wave?
Wave Properties
The Parts of a Wave
• Waves can differ in how much
energy they carry and in how fast
they travel.
• Waves also have other
characteristics that make them
different from each other.
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Wave Properties
The Parts of a Wave
• crests - Alternating high points
• troughs - Alternating low points
Wave Properties
The Parts of a Wave
• Parts of a compressional wave
• Compressionregion where
the coils are
close together.
Wave Properties
The Parts of a Wave
• Rarefaction- region where coils are spread
apart
Wave Properties
Wavelength
• wavelength is the distance between
two of the same points on a wave
Wave Properties
Wave Properties
11.2
Wavelength
Wavelength
• The wavelengths of sound waves
that you can hear range from a few
centimeters for the highest-pitched
sounds to about 15 m for the
deepest sounds.
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Wave Properties
Frequency and Period
• frequency of a wave is the
number of wavelengths that
pass a fixed point each second.
• transverse waves- number of
crests that pass by a point each
second.
Wave Properties
Frequency and Period
• Period- amount of time it takes one
wavelength to pass a point.
• Frequency is expressed in hertz
(Hz).
Wave Properties
Wavelength is Related to Frequency
• As frequency increases, wavelength
decreases.
• If you move the
rope up, down,
and back up in 1
s, the frequency
of the wave you
generate is 1 Hz.
Frequency
 Def: periodic
motion is any motion that repeats
at regular time intervals
 Def: Frequency is how many waves pass by in a
given time
 Frequency = 1/period = 1 / T = f
 It is measure in hertz (Hz) which is 1/s where s
is seconds
 Humans can hear 20Hz to 20000Hz
 A wave’s frequency equals the frequency of the
vibrating source producing the wave
Wave Properties
Sound and Mediums
• Sound travel faster in liquids and
solids than they do in gases.
• Light waves travel more slowly in
liquid and solids than they do in
gases or in empty space.
• Sound waves faster in a material if
the temperature of the material is
increased.
Problems involving f, λ (wavelength)
& speed
 Def: Wavelengths
of waves are measured from
one crest to the next (crest to crest OR
trough to trough) or from one compression
to the next
 It is represented by the Greek letter lambda
whose symbol is λ
 The period of a wave is how long it takes for
a complete wave to go by a spot, symbol is T
 Increasing the frequency (f) of a wave
decreases the wavelength (λ)
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•
•
•
•
•
•
Recall that speed = distance / time
For waves we can use wavelength for distance
(in meters, m) and period for time (s for
seconds)
Speed = wavelength X frequency
v=λ*f
The speed of a wave depends on what it travels
through (medium)
If the medium particles are closer together the
energy from the wave can make vibrations easier
So waves travels best in solids, then liquids, and
worse in air
Amplitude and Energy
Def: amplitude is the difference between crest
and the rest position or point of origin
 Def: the rest position or point of origin is an
imaginary line through the middle of the wave
that separates the crest from the trough
 The more energy a wave has, the greater its
amplitude

Wave Properties
Calculating Wave Speed
• You can calculate the speed of a
wave represented by v by multiplying
its frequency times its wavelength.
Wave Properties
Amplitude and Energy
• Amplitude energy carried by a
wave.
• The greater the
wave’s amplitude
is, the more energy
the wave carries.
Click image to play movie
Wave Properties
Amplitude of
Compressional Waves
• The closer
the coils
are in a
compressi
on, the
farther
apart they
are in a
rarefaction.
Wave Properties
Amplitude of Transverse
Waves
• Draw Picture
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Section Check
Question 1
If a wave has a high point and a low
point, is it a compressional or
transverse wave?
Section Check
Answer
Transverse
waves have
alternating
high points,
called crests,
and low
points, called
troughs.
Section Check
Question 2
What is the wavelength of a wave?
Section Check
Answer
A wavelength
is the distance
between one
point on a
wave and the
nearest point
just like it.
Section Check
Section Check
Question 3
Answer
Which of the following refers to the
number of wavelengths that pass a
fixed point each second?
A. frequency
B. period
C. wavelength
D. wave speed
The answer is A - Frequency. Period
is a length of time, and wavelength is
a distance.
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The Behavior of Waves
17.3 Behavior of Waves
To Be a Wave
 Objectives:
 1. Explain
reflection and refraction and how
they affect waves
 2. Identify several factors that affect the
amount of wave diffraction
 3. Discuss two types of interference
 4. Explain what a standing wave is and detail
the wavelengths that produce it
• All waves must- Reflect, Refract,
and Diffract.
The Behavior of Waves
Reflection and Refraction
•
•
•
•
•
Def: Reflection occurs when a wave meets a
boundary and bounces off
The wave cannot pass through the surface
Reflection does not change the speed or
frequency of a wave, but the wave can be flipped
upside down
Def: Refraction is the bending of a wave as it
travels through different mediums
When a wave enters a medium at an angle,
refraction occurs because one side of the wave
moves more slowly than the other side (ex:
pencil in water)
Reflection
• How does the reflection
of light allow you to see
yourself in the mirror? It
happens in two steps.
First, light strikes your
face and bounces off.
Then, the light reflected
off your face strikes the
mirror and is reflected
into your eyes.
•
What about Sound Waves
The Behavior of Waves
Echoes
• Echo - sound waves hit an object,
they reflect and come back to you.
You hear the sound multiple times.
The Behavior of Waves
The Law of Reflection
Draw this Picture:
• The beam striking
the mirror is
called the incident
beam.
• The beam that
bounces off the
mirror is called
the reflected
beam.
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The Behavior of Waves
The Law of Reflection
• The line
drawn
perpendicul
ar to the
surface of
the mirror
is called
the normal.
The Behavior of Waves
The Law of Reflection
• law of reflectionThe angle of
incidence is
equal to the
angle of
reflection.
• All reflected
waves obey this
law.
The Behavior of Waves
Refraction
• RefractionBending of a wave
caused by a
change in its
speed as it moves
from one medium
to another.
The Behavior of Waves
The Law of Reflection
• The angle formed
by the incident
beam and the
normal is the
angle of
incidence.
• The angle formed
by the reflected
beam and the
normal is the
angle of refection.
The Behavior of Waves
Refraction
• When a wave passes from one
medium to another. It changes speed
and direction. (Example: from air to
water)
• If the wave is traveling at an angle
when it passes from one medium to
another, it changes direction, or bends,
as it changes speed.
The Behavior of Waves
Refraction of Light in Water
• Light waves travel slower in water than
in air. This causes light waves to
change direction and bend toward the
normal
• When light waves
travel from air to
water, they slow
down and bend
toward the normal.
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The Behavior of Waves
The Behavior of Waves
Refraction of Light in Water
Refraction of Light in Water
• You may have noticed that objects that
are underwater seem closer to the
surface than they really are.
• In the figure, the light
waves reflected from
the swimmer’s foot
are refracted away
from the normal and
enter your eyes.
• However, your brain assumes that all
light waves have traveled in a straight
line.
• The light waves that enter your
eyes seem to have come from a
foot that was higher in the water.
The Behavior of Waves
Diffraction and Interference
 Def: Diffraction
is the bending of a
wave around an obstacle
 A wave diffracts more if its wavelength
is large compared to the size of an
opening or obstacle
 The two types of interference are
constructive interference and
destructive interference
The Behavior of Waves
Diffraction
• Diffraction - an object causes a wave
to change direction and bend around
it.
• Diffraction and refraction both cause
waves to bend. The difference is that
refraction occurs when waves pass
through an object, while diffraction
occurs when waves pass around an
object.
Diffraction
• When waves strike an object, several
things can happen.
• The waves can bounce off, or be reflected.
• If the object is transparent, light waves can
be refracted as they pass through it.
• Waves also can behave another way
when they strike an object. The waves
can bend around the object.
The Behavior of Waves
Diffraction
• After they pass
through the
opening, the
waves spread
out.
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The Behavior of Waves
Diffraction and Wavelength
• Examples of diffraction?
The Behavior of Waves
Diffraction of Radio Waves
• AM radio waves have longer
wavelengths than FM radio waves do.
Because of their longer wavelengths,
AM radio waves diffract around
obstacles like buildings and
mountains.
The Behavior of Waves
Hearing Around Corners
• Light waves have a much shorter
wavelength. They are hardly
diffracted at all by the door.
The Behavior of Waves
Interference
• interference
When two or
more waves
overlap and
combine to form
a new wave
• As a result, AM radio reception is often
better than FM reception around tall
buildings and natural barriers such as
hills.
The Behavior of Waves
Constructive Interference
The Behavior of Waves
Constructive Interference
• constructive interference- waves add
together.
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The Behavior of Waves
Destructive Interference
• destructive interference- waves cancel
each other.
Standing Waves
 Interference may
cause standing waves Def: Standing waves appear not to
move along the medium
 Has areas of destructive interference
where there is no vibration called
nodes
 Def: a node is a point on a standing
wave that has no displacement from the
rest position
The Behavior of Waves
Standing Waves
• standing wave - waves equal in
wavelength and amplitude, travel in
opposite directions, and interfere with
each other.
 The
combo of two or more waves at
the same place at the same time causes
interference
 Def: When two crests meet the
interference is constructive-the wave
becomes more energetic-the amplitude
increases
 Def: When a crest and a trough meet
they cancel each other and decrease
the amplitude –destructive
 Areas
of maximum interference
called antinodes
 Def: an antinode is a point where a
crest & trough meet midway
between 2 nodes
 A standing wave forms only if half a
wavelength or a multiple of half a
wavelength fits exactly into the
length of a vibrating cord
The Behavior of Waves
Standing Waves in Music
• When the string of a violin is played with
a bow, it vibrates and creates standing
waves.
• Some
instruments,
like flutes,
create standing
waves in a
column of air.
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The Behavior of Waves
Section Check
Resonance
• resonance - object is made to vibrate
by absorbing energy at its natural
frequency
• If enough energy is absorbed, the
object can vibrate so strongly that it
breaks apart.
Question 1
State the law of reflection.
Answer
According to the law of reflection, the angle
of incidence is equal to the angle of
reflection.
Section Check
Section Check
Question 2
Answer
__________ is the bending of a wave
caused by a change in its speed as it
moves from one medium to another.
The answer is C. The greater the change in
speed is, the more the wave bends.
A.
B.
C.
D.
diffraction
diffusion
refraction
reflection
Section Check
Section Check
Question 3
Answer
Which is the means by which you can hear
around corners?
The answer is A. Diffraction occurs when
an object causes a wave to change
direction and bend around it.
A.
B.
C.
D.
diffraction
diffusion
reflection
refraction
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