Chapter 25
Vibrations & Waves
Helpful website:
http://www.kettering.edu/~drussell/demos.html
Vibration vs. Wave
•Vibration
A “wiggle” in time.
•Wave
A “wiggle” in space and time.
Pendulum Equation
T


= 2π √ L/g
Period = 2π √ length of pendulum
acc. of gravity
s=
m
m/s2
Sine Curve
Crest
Wavelength

Trough
Amplitude
A
Sine Curves
 Amplitude
- distance from the baseline to
the crest of a wave
 Wavelength
- the distance from the top of
one crest to the top of the next one.

Symbol used:
λ
“lambda”
Period

Period - the time required
for 1 vibration
• measured in seconds

The period does not
depend on the amount of
mass.

The period does depend on
the length of the pendulum.
1
T
f
Frequency

Frequency - # of vibrations per second
• measured in Hertz
1
f 
T
Waves
Waves
transmit energy.
•Electromagnetic Waves
•Do not require a medium to
propagate.
• Examples:
•AM and FM radio waves
•Light
Mechanical Waves
• Require a medium in which to
propagate.
•Examples:
•Water
•Sound
Which travels faster,
light or sound?
Wave Motion
 medium
- the stuff that carries the wave
Waves
Medium
water waves
water
waves on a rope
rope
stadium waves
people
sound
air
light
space (vacuum)
Wave Speed
 The
speed with which waves pass by a
particular point.
• e.g. the speed of a surfer
 Depends
only on the type of medium.
 Remember,
v = d/t
Waves on a Rope
Wave Speed
v = f
Wave speed = (wavelength)(frequency)
m/s = (m)(Hz)
*
If a water wave oscillated up and down
three times each second and the
distance between wave crest is 2 m…
What is its frequency?
Answer: 3 Hz
What is its period?
Answer: 1/3 second
What is its wavelength?
Answer: 2 m
What is its wave speed?
Answer: 6 m/s
Types of Waves
We will study 2 types of mechanical waves
1)Transverse Waves
2)Longitudinal Waves
Transverse Waves
 Side
to side vibration in a direction
perpendicular to the wave's motion.
 Examples:



water waves
waves on a rope
string musical instruments
Longitudinal Waves
 Back
and forth vibration in a direction parallel to
the wave's motion.
 Examples:



slinky waves
sounds waves
Ultrasound waves
• (sound at a frequency which
is outside of the range of human hearing)
Transverse & Longitudinal Waves
 Earthquakes
Compression Waves
(Primary Waves: P-waves
Transverse Waves
(Shear Waves: S-waves
have both!
Water Waves
Wavelength increases
the farther away from
the source it travels.
Example Question
*
The distance from trough to trough on a
periodic wave is called its...
a) frequency.
 b) period.
 c) wavelength.
 d) amplitude.

Example Question
*
Changing which of the following affects
the period of a pendulum?
a) mass
 b) amplitude
 c) length

 d) angle

Example Question
*
Dipping a finger in water faster and faster
causes the wavelength of the spreading
waves to
a) increase
 b) decrease
 c) stay the same
 d) not enough information is given

INTERFERENCE
 Constructive
or destructive interference
results when waves add.
 Standing
Waves - wave pattern
produced from interfering waves

Examples
•
•
•
•
•
Rope Waves
Vibrating Strings in Lab
Guitar Strings
Bell Wave Machine
Organ Pipe
DOPPLER EFFECT

the change in wavelength due to motion of
the source

"Wheeeeeeeeeeee…….Oooooooooooooo”

Examples:



moving cars and trains
moving buzzer in a nerf ball (in class)
rotating whistle
Draw Doppler Picture
Example Question
*
A train whistle at rest has a frequency of
3000 Hertz. If you are standing still and
observe the frequency to be 3010 Hertz,
then you can conclude that...
a) the train is moving away from you.
 b) the train is moving toward you
 c) the sound from the whistle has echoed
 d) not enough information is given

Shock Wave
The cone-shaped wave
made by an object moving at
supersonic
speed
through
a

fluid.
(Here, the source is moving
faster than the wave speed,
which is the speed of
sound!!)
Sonic Boom

Piled up
wave
fronts
produce a
shock
wave
Plane
End of Section...