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WAVES AND VIBRATIONS NOTES

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WAVES AND VIBRATIONS NOTES * A wave is a vibration ... so lower sound Describe the frequency of vibration and ... When the vibration of one object causes ... – PowerPoint PPT presentation

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Title: WAVES AND VIBRATIONS NOTES


1
WAVES AND VIBRATIONS NOTES
2
A wave is a vibration moving in time and space  
It cannot exist in one place but must
extend from one place
to another
Wave motion is a way to transfer energy without
transfer of
matter
3
A vibration is back and forth movement
A single disturbance or vibration is a wave
pulse If the source repeatedly vibrates it
produces periodic motion
4
Sound is produced when matter vibrates
Demonstration What happens when a tuning fork is
placed in water?
Since sound is produced when matter vibrates,
sound is a form of mechanical energy.
5
Crest
midpoint
trough
Crest High point in the wave Trough Low point
in the wave
--------Midpoint of wave where the wave is in
equilibrium
Amplitude Distance from the midpoint to the crest
or trough The higher the amplitude the stronger
the wave
6
Wavelength is the distance from one point on the
wave to the identical point on the next wave.
Crest to Crest, or trough to
trough
7
 Frequency is How frequently a vibration occurs.
How many wavelengths or wave cycles pass in one
second.
If 2 wavelengths (or vibrations, or wave cycles)
pass a point in one second, then the frequency
is
2 vibrations/ second. (Or 2 cycles/ sec)
8
Hertz is a unit of frequency (abbreviated
Hz)   It was named after Heinrich Hertz who
demonstrated radio waves in 1886.
One vibration per second is one Hertz.   2
vibrations (or waves, or cycles) per second is
________Hertz
2
9
DEMONSTRATIONS Hacksaw blade (hold blade
against table edge. Change length hanging over
and vibrate it. Describe the frequency of
vibration and pitch (highness or lowness of the
sound) when the hacksaw blade is long.
Longer vibrations, so lower sound
Describe the frequency of vibration and pitch
when the hacksaw blade is shorter.
Higher vibrations so higher pitch
10
Containers with water. (pop bottles to blow
across or crystal glass to rub finger
around)  What is the relationship between the
amount of water in each container and the pitch?
Less water vibrating means higher pitch.
More water vibrating is lower pitch
The longer the length of vibrating air above the
water, the Lower the pitch when blowing into
a bottle
11
If a wave's frequency is known, then you can
calculate the period of time it takes for one
wave to pass.   If the frequency is 2 Hz, then
the time it takes for 1 vibration is ½
second .  
Explain the relationship between Period and
Frequency
Frequency 1 Period
1 Period (T)
Frequency It is an inverse relationship If
freq 5 Hz then Period 1/5 or .2 seconds
12
The Sears Building in Chicago sways back and
forth at a vibration frequency of 0.1 Hz. What
is its Period of vibration?
T 1 / f 1 / 0.1 cycles/sec 10
seconds/cycle It takes 10 seconds for it
to sway back and forth one time
13
WAVE SPEED Speed of a wave depends on the medium
through which the wave travels. Speed depends on
  • Density of materials how many molecules there
  • are to bump into each other
  • Elasticity how well it changes shape and then
  • springs back. More elastic, the faster the
  • waves travel.
  • Temperature higher the temperature,
  • the faster the molecules move

Sound travels about 330 m/s - 350 m/s depending
on the temperature of the air. It travels 4
times faster in water, and 15 times faster in
steel.
14
 
Wave Speed frequency x wavelength
? Greek letter Lambda
wavelength
Wave Speed
Freq.
?
15
Sound Waves Frequency (Hz) Wavelength (m)
Wave Speed (m/s) 160 2.13
340 264
1.29 340
396 0.86
340 528
0.64 340
Sound waves at a concert reach your ears at the
same time. Lower notes have long waves, high
notes (higher frequency) have shorter wave
lengths, but they all have the same wave speed.
16
If a water wave vibrates up and down 2 times each
second and the distance between wave crests is
1.5 meters, what is the frequency of the wave?
What is its wavelength? What is its speed?
F 2 Hz Wavelength 1.5 m
Speed 2 cycles x 1.5 m 3 m/sec
Sec Cycle
17
What is the wavelength of a 340 Hz sound wave
when the speed of sound in air is 340 m/s?
Wavelength speed 340 m/sec 1
meter/cycle Frequency
340 cycles/sec
1 miles 1609 meters. If the speed of sound is
340 m/s, how long does it take for thunder to
travel 1 mile?
T distance/speed 1609 m 4.7
seconds 340 m/sec So about 5
seconds/mile
18
TRANSVERSE WAVES Motion of the wave is at
right angles to the direction of travel
Examples waves on the surface of liquids,
Radio waves, light waves slinky demo
19
LONGITUDINAL WAVES motion of waves is in the
same direction
20
Examples sound waves
SMOKE RINGS
21
AS A REVIEW OF THE WAVE TYPES
Transverse
rarefaction
Longitudinal
compression
22
NATURAL FREQUENCY When you drop a wrench and
baseball bat on the floor, you hear 2 different
sounds. They vibrate differently. When any
object composed of an elastic material is
disturbed it vibrates at its own special set of
frequencies. This natural frequency depends on
factors such as
elasticity and the shape of the object
RESONANCE When the vibration of one object
causes another object to vibrate at its natural
frequency A dramatic increase in Amplitude
occurs.
23
Examples of Resonance Swinging on a swing pump
in rhythm with natural frequency of the swing,
go higher and higher
Tuning fork can cause another to vibrate
Tacoma Narrows Bridge destroyed by a 40 mph
wind (video)
Wine glass shattering
24
INTERFERENCE OF WAVES
Constructive Interference when one wave crest
overlaps another crest and they build together
Animation courtesy of Dr. Dan Russell, Kettering
University
25
  Destructive Interference when one wave crest
overlaps a trough and they cancel each other
out.
USES noise canceling earmuffs for pilots, jack
hammers
Animation courtesy of Dr. Dan Russell, Kettering
University
26
SHOW MOIRE PATTERNS
27
Standing Waves when wave is reflected back
exactly opposite to the original wave
A anti-node B node
Animation courtesy of Dr. Dan Russell, Kettering
University
28
Beats periodic variation in loudness of sounds
caused when 2 slightly different frequencies
are sounded together
demo
Animation courtesy of Dr. Dan Russell, Kettering
University
If one tuning fork vibrates at 264 Hertz, and the
other at 262 Hertz, then they are in step 2
times each second. A beat frequency of 2 hz
is heard.
29
THE DOPPLER EFFECT
The Doppler effect is a change in frequency of a
wave due to the motion of the source or
receiver. Example change in pitch of a car
engine, horn or siren as it passes you
As a sound wave approaches you, the pitch is
higher than normal because the waves Come
faster, at a higher frequency   After it passes,
the sound waves are farther apart so the sound
is Lower
30
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31
Doppler Effect occurs for visible light
too.   Short lights waves are Blue and
Long wavelengths are Red Astronomers can
measure whether stars and galaxies are moving
toward us or away from us by looking at the
light shift.  Blue Shift means - the light is
coming faster stars are moving forward Red
Shift means- the star is going away
They can calculate the speed of a spinning star.
There is a blue shift on the side spinning
toward us and a red shift on the side spinning
away.
32
Police use the Doppler effect of radar to
determine the speed of a car. A computer in the
radar gun compares the frequency of the radar
emitted by the antenna with the frequency of
the reflected waves.
33
OSCILLOSCOPE A device that is used to measure
electrical signals as waves. It shows the
strength of the wave (amplitude) and the period
of time between each wave. You can then
calculate the frequency of the waves.
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