What is a wave?

Take a moment to answer this question
with the students at your table.

As a group, create a definition and give
an example of a wave.
(Be a bit more original than ‘an ocean wave.’
I know you can think of a better example…)
a wave
is a rhythmic disturbance that
carries energy through matter or
space
Mechanical Waves

require a medium

medium: the matter through which a
wave travels

examples: water, air, anything made of matter
waves transfer energy from one
point to another with little or
no permanent displacement to
the particles in the medium
Two Types of Mechanical
Waves:
1.
Transverse Waves
2.
Longitudinal Waves
Transverse Waves

waves that propagate (or move)
perpendicular to the direction of travel
crest

the point on the medium that exhibits the
maximum amount of positive or upward
displacement from the rest position
trough

the point on the medium that exhibits the
maximum amount of negative or
downward displacement from the rest
position
amplitude

the maximum displacement of a particle
on the medium from its equilibrium
position
wavelength

the length of one wave is measured
from a point on one wave to the same
point on the next wave
 trough to trough
 crest to crest
 midpoint to midpoint
Anatomy of a Transverse Wave

Label the following parts of a transverse
wave in your notes:
 equilibrium
 crest
 trough
 amplitude
 wavelength
Longitudinal Waves

waves that propagate (or move) parallel
to the direction of motion
compression

a point on a medium through which a
longitudinal wave is traveling that has
the maximum density
rarefaction

a point on a medium through which a
longitudinal wave is traveling that has
the minimum density
Anatomy of a Longitudinal Wave

Label the following parts of a
longitudinal wave in your notes:
 compression
 rarefaction
 wavelength
Describing a Wave

frequency: the number of vibrations an
object makes in a unit of time (generally
one second)
 the unit of frequency is Hz

period: the time needed to complete one
vibration (measured in seconds)

The Sears Tower sways back and forth at
a frequency of about .1 Hz.
What is the period of vibration?
Speed Review

an object’s speed is described by the
distance traveled in a specific amount of
time (generally measured in m/s)
Wave Speed
v=λ●f
wave speed = wavelength ● frequency

A wave with a frequency of 60.0 Hz
travels through vulcanized rubber with a
wavelength of 0.90 m.
What is the speed of this wave?

The lowest pitch that the average
human can hear has a frequency of
20.0 Hz.
If sound with this frequency travels through
air with a speed of 343 m/s, what is its
wavelength?

A ship anchored at sea is rocked by
waves that have crests 14 m apart. The
waves travel at 7.0 m/s.
How often do the wave crests
reach the ship?

Earthquakes generate shock waves that
travel through Earth’s interior to other parts
of the world. The fastest of these waves are
longitudinal waves, like sound waves, and
are called primary waves, or just p-waves. A
p-wave has a very low frequency, typically
around 0.050 Hz.
If the speed of a p-wave with this frequency is
8.0 km/s, what is its wavelength?

A dolphin can typically hear sounds with
frequencies up to 150 kHz.
What is the speed of sound in water if a
wave with this frequency has a wavelength
of .01 m?

Sonar is a device that uses reflected sound
waves to measure underwater depths.
There is a sonar signal that has a
frequency of 288 Hz. If the speed of sound
in water is 1.45x103 m/s,
what is the wavelength of
the sonar signal?
Cicadas produce a buzzing sound that has a
wavelength in air of 2.69 m. If the speed of
sound in air is 343 m/s,
what is the frequency of the sound produced
by a cicada?
What is its period?
A hiker shouts toward a vertical cliff 685 m
away. The echo is heard 4 s later.
What is the speed of sound of the hiker’s voice
in air?
The wavelength of the sound is .75 m. What is
its frequency?
What is the period of the wave?
Wave Behavior

What happens when a ball is thrown
against a wall?

Predict what would happen if a wave
encounters a rigid boundary?

When a wave pulse is sent down a medium
connected to a rigid wall,
the energy transmitted
is reflected.

The reflected wave
is inverted due to
Newton’s Third Law of
Motion.

Predict what would happen if two waves were
in the same place on a medium.

Principle of Superposition
 the displacement of a medium caused by two or
more waves is the algebraic sum of the
displacements of the individual waves
 two or more waves can combine to form a new wave

the result of the superposition of two or more
waves is called interference
Two Types of Interference
1.
2.
Constructive
Destructive
Constructive Interference

occurs when wave displacements are in
the same direction

the two waves pass through each other
without changing their shapes or sizes

the resultant pulse at the overlap of the
two waves is the algebraic sum
Constructive Interference
Destructive Interference

the superposition of waves with equal
but opposite amplitudes

when the pulses meet at the same
location, the displacement of the
medium is zero

just as constructive interference, the
waves pass through one another
unchanged
Destructive Interference
Standing Waves

a wave pattern that remains in a
constant position

the result of reflection and wave
interference

characterized by two fixed points on the
wave
Tacoma Narrows Bridge

In an effort to save money, corners were
cut in the design of the bridge thus
compromising its stability

On November 7, 1940 the Tacoma
Narrows suspension bridge collapsed
due to 42 mph wind induced vibrations.