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# Waves Carry Energy

Basic information on waves: what they do, how they are made and how we use them.
by

## Joshua Boyd

on 5 January 2012

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#### Transcript of Waves Carry Energy

Waves
always carrying energy!!!!
Simple harmonic motion
Exemplified by a spring or pendulum

it is a periodic back and forth action

repetitive...frequency stays the same
Simple harmonic motion described
What is the frequency of a pendulum that completes 2 swings in a second?

What is the frequency of a pendulum that completes 1000 swings per second?

What is the frequency of a pendulum that completes half a swing per second?
How are these two related?
Period (T)
Frequency (f)
A pendulum has a specific frequency

Frequency is the number of complete swings in a given amount of time

Hertz (Hz) is the number of complete cycles per second
There are two related terms used to describe SHM
The total amount of time required for a cycle

A cycle would be a complete back and forth swing
Period is the inverse of frequency
T=1/f
Frequency is the inverse of Period
f=1/T
If a kid spins in his chair 5 times per second what is his frequency? what is the period of his rotation?
If a girl hops over a jump rope every 2 seconds what is the frequency of the jump ropes swing? what is the period of the jump ropes swing?
The Sears tower oscillates at a rate of 0.1 Hz, how long does it take to do a complete back and forth swing?
SHM graphed
Simple harmonic motion graphed produces a sine curve
A sine curve is a picture of a wave
The anatomy of a wave:
To determine speed we need to know two things:
Calculating wave speed!!!!
Time
Distance
Time with waves is communicated by the frequency
Distance is found by using the wavelength
Wave speed = wavelength x frequency
or
V = f
What is the speed of a wave when the crests are 10m apart and the waves have a frequency of .5Hz?

What is the speed of a wave with a wavelength of .1m and a frequency of 20 waves per second?

What is the wavelength of a sound that travels at 340m/s and has a frequency of 256 Hz?
There are two types of waves
Transverse Waves

input of energy is at a 90 degree angle to the medium

Obvious crests and troughs

Most commonly thought of waves

Longitudinal waves

input of energy is in line (180 degree) with the medium

Instead of crests and troughs there are compressions and rarefactions

Sound waves, shock waves....
What is it that travels in a wave?

How does it travel?

What happens if waves collide?
Waves that collide interfere with one another!!!

just like people.....we can cooperate...or fight

The two types of wave interference are Constructive and Destructive
Interference!!!!
Frequency
Period
This region of light gray represents a trough
This region of light gray represents a trough
this region of dark gray represents a crest
this region of dark gray represents a crest
this band is an area where crests meet crests and troughs meet troughs
this band is an area where crests meet crests and troughs meet troughs
This area is the location where crests meet troughs and troughs meet crests
This area is the location where crests meet troughs and troughs meet crests
Vibrations / energy is put in here
When a crest encounters a crest or a trough encounters a trough the energies pass through eachother making bigger crests or deeper troughs
When a trough encounters a crest (or vice versa) the energies pass through eachother and are cancelled out in the medium
If these were sound waves, these regions would be louder!!!! (constructive)
These regions would be quieter
(destructive)

Can waves interfere with one another?

Crest encounters trough......

Trough encounters trough....

What if this happens on a spring???

Send a wave down a spring...

When the reflected crest comes back and you match it with another crest you get a bigger wave

It is possible to match the frequency of the spring (based on length, tension and mass) so that a standing wave develops.

Standing waves are responsible for producing tone in string, and some wood wind instruments.
Standing waves....
Node. Region of destrucive interference
Antinode. Region of constructive intereference
What if the source of a wave moves?
Will it affect the waves produced?
Will it affect the waves recieved?
Here we see a stationary vibration in a medium... producing waves that propogate in concentric circles
Here we notice the the source of the vibrations is moving through the medium at a speed that is less than the speed of the wave. Notice, the waves in front of the source are closer together... and the ones in the back......What would this sound like?
This in known as the doppler effect

Police can tell how fast you are going

Meteorologists can tell where clouds are going

Military can pick up presence of aircraft
Here we see that the speed of the emitter is the same as the speed of the wave.

This produces a large amount of constructive interference at the front of the emitter. this is known as a bow wave
Here we see the emitter is faster than the speed of a wave....

This produces a shock wave

Sonic booms are created this way
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