Contributed by:
Mechanical waves, Properties of mechanical waves, Behaviour of waves, Standing waves, Sound and hearing, Doppler's effect
1.
Chapter 17:
Mechanical
Waves and Sound
Jennie L. Borders
2.
Warm-Up Nov. 4
1. What is a wave?
2. What is the highest
point on a wave called?
3. What is reflection?
3.
Section 17.1 –
Mechanical Waves
A mechanical wave is a disturbance in
matter that carries energy from one
place to another.
Mechanical waves require matter to
travel through.
The material through which a wave
travels is called a medium.
A mechanical wave is created when a
source of energy causes a vibration to
travel through a medium.
4.
Types of Mechanical
Waves
The three main types of mechanical
waves are transverse waves,
longitudinal waves, and surface
waves.
A point on the wave vibrates up and
down between a maximum and
minimum height.
5.
Transverse Waves
A transverse wave is a wave that
causes the medium to vibrate at
right angles to the direction in which
the wave travels.
6.
Transverse Waves
The highest point of the wave above
the rest position is the crest.
The lowest point below the rest
position is the trough.
7.
Longitudinal Waves
A longitudinal wave is a wave in which
the vibration of the medium is parallel
to the direction the wave travels.
An area where the particles in a
medium are spaced close together is
called a compression.
An area where the particles in a
medium are spread out is called a
rarefaction.
8.
Surface Waves
A surface wave is a wave that travels
along a surface separating two
media.
The motion of the particles is in a
circle.
9.
Section 17.1 Assessment
List the three main types of
mechanical waves.
For each type of wave, compare the
vibration of the medium to the
direction of the wave.
10.
Warm-Up Nov. 6
1. What are the 3 types of waves?
2. How is a wave made?
3. What is the lowest point of the
wave called?
11.
Section 17.2 – Properties of
Mechanical Waves
Any motion that repeats at regular
time intervals is called periodic
motion.
The time required for one cycle, a
complete motion that returns to its
starting point, is called the period.
12.
Frequency
Any periodic motion has a
frequency, which is the number of
complete cycles in a given time.
Frequency is measured in cycles per
second, or hertz (Hz).
A wave’s frequency equals the
frequency of the vibrating source
producing the waves.
13.
Wavelength
Wavelength is the distance between
a point on one wave and the same
point on the next cycle of the wave.
Increasing the frequency of a wave
decreases the wavelength.
14.
Wave Speed
Formula for the speed of waves
Speed = Wavelength x Frequency
v=x
Speed = m/s
Wavelength = m
Frequency = 1/s or Hz
15.
Sample Problem
One end of a rope is vibrated to
produce a wave with a wavelength of
0.25m. The frequency of the wave is
3.0Hz. What is the speed of the wave?
v=x
v=? v=x
= 0.25m v = 0.25m x 3.0(1/s)
= 3.0Hz v = 0.75m/s
16.
Practice Problems
A wave on a rope has a wavelength
of 2.0m and a frequency of 2.0Hz.
What is the speed of the wave?
v=x v = 2.0m x 2.0(1/s) = 4.0m/s
A wave in the water has a frequency
of 4Hz and a wavelength of 0.1m.
What is the speed of the wave?
v=x v = 0.1m x 4(1/s) = 0.4m/s
17.
Practice Problems
What is the wavelength of an
earthquake wave if it has a speed of
5km/s and a frequency of 10Hz?
=x = v/ = 5km/s / 10(1/s) = 0.5k
A wave travels at 1.8m/s and have a
wavelength of 1.2m. What is the
frequency of this wave?
= x = v/ = 1.8m/s / 1.2m = 1.5H
18.
Speed
If you assume that waves are
traveling at a constant speed, then
wavelength is inversely proportional
to frequency.
If wavelength increases, then
frequency decreases.
If wavelength decreases, then
frequency increases.
19.
Amplitude
The amplitude of a wave is the
maximum displacement of the
medium from its rest position.
The more energy a wave has, the
greater is its amplitude.
20.
Section 17.2 Assessment
How is wavelength related to
frequency for waves moving at a
constant speed?
How is the energy of a wave related
to its amplitude?
If you double the frequency of a
wave, what is the effect on its
wavelength?
21.
Section 17.2 Assessment
A wave on a rope has a frequency of
3.3Hz and a wavelength of 1.2m.
What is the speed of the wave?
v = x v = 1.2m x 3.3(1/s) = 4.0m/s
22.
Warm-Up Nov.7
1. What is the area of a
longitudinal wave where the
particles are close together?
2. How do the particles of a
surface wave move?
3. What is the lowest point on a
transverse wave called?
23.
Section 17.3 – Behavior
of Waves
Reflection occurs when a wave
bounces off a surface that is cannot
pass through.
Reflection does not change the
speed or frequency of a wave, but
the wave can be flipped upside
down.
24.
Refraction
Refraction is the bending of a wave
as it enters a new medium at an
angle.
When a wave enters a medium at an
angle, refraction occurs because one
side of the wave moves more slowly
than the other side.
25.
Diffraction
Diffraction is the bending of a wave
as it moves around an obstacle or
passes through a narrow opening.
A wave diffracts more if its
wavelength is large compared to the
size of an opening or obstacle.
26.
Interference
Interference occurs when two or
more waves overlap and combine
together.
Two types of interference are
constructive interference and
destructive interference.
27.
Constructive
Interference
Constructive Interference occurs
when two or more waves combine to
produce a wave with a larger
displacement.
28.
Destructive Interference
Destructive interference occurs
when two or more waves combine to
produce a wave with a smaller
displacement.
29.
Standing Waves
A standing wave is a wave that
appears to stay in one place.
A node is a point on a standing wave
that has no displacement from the
rest position.
An antinode is a point where a crest
or trough occurs midway between
two nodes.
30.
Section 17.3 Assessment
How is a wave changed by reflection?
What causes refraction when a wave
enters a medium at an angle?
What determines how much a wave
diffracts when it encounters an opening
or an obstacle?
How does the frequency of a reflected
wave compare with the frequency of
the incoming wave?
What is the amplitude of a wave that
results when two identical waves
interfere constructively?
31.
Warm-Up Nov. 11
1. What is reflection?
2. What is refraction?
3. What is diffraction?
32.
Section 17.4 – Sound and
Hearing
Sound waves are longitudinal waves
that travel through a medium.
Many behaviors of sound can be
explained using a few properties –
speed, intensity and loudness, and
frequency and pitch.
33.
Speed
In dry air at 20oC, the speed of sound
is 342 m/s.
In general, sound waves travel
fastest in solids, slower in liquids,
and slowest in gases.
This is due to the fact that particles
in a solid tend to be closer together
than particles in a liquid or a gas.
34.
Intensity and Loudness
Intensity is the rate at which a wave’s
energy flows through a given area.
Sound intensity depends on both the
wave’s amplitude and the distance from
the sound source.
The decibel (dB) is a unit that compares
the intensity of different sounds.
Loudness is a physical
response to the intensity
of sound.
35.
Frequency and Pitch
Pitch is the frequency of a sound as
you perceive it.
High-frequency sounds have a high
pitch, and low-frequency sounds
have a low pitch.
36.
Ultrasound
Ultrasound is sound at frequencies
higher than most people hear.
Ultrasound is used in a variety of
applications, including sonar and
ultrasound imaging.
37.
Sonar
Sonar is a technique for determining
the distance to an object
underwater.
Sonar stands for sound navigation
and ranging.
38.
Doppler Effect
The Doppler effect is a change in
sound frequency caused by motion of
the sound source, motion of the
listener, or both.
As a source of sound approaches, an
observer hears a higher frequency.
When the sound source moves away,
the observer hears a lower frequency.
39.
Section 17.4 Assessment
List five properties used to explain
the behavior of sound waves.
Names two uses for ultrasound.
What is the Doppler effect?
If workers in a distant stone quarry
are blasting, why can you feel the
explosion in your feet before you
hear it?
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