/home/davie/Public/PHYSICS/waves 1&2/Waves2.ppt

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Waves

Medium – The material that a wave travels through (air, water, rope, etc.)

What is the medium for earthquake waves?

Waves Carry Energy!!!

Waves are transmission of disturbances!!!

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• Vibration

–A repeating back-and-forth motion

• Wave

–A vibration that travels

–Some waves require material media while others do not

OR

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Wave Motion

Waves transfer energy, NOT matter.

When someone speaks, they make the air vibrate back and forth, but the air particles do not move from the speaker all the way to the

listener’s ear.

When you drop a rock in a pond, the water moves up and down, but not forward.

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Wave Motion

The energy transferred from a vibrating object is carried by a disturbance in the medium (air, water etc.), not by moving

particles of matter.

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Categories of Waves

Examples:

1.radio waves

2.Micro waves

3.Radian heat

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Categories of Waves

Examples:

1.Water waves

2.Sound waves

3.Waves on a spring or a string

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Wave Typesa) Transverse Wave – The motion of the particles is at a right angle (perpendicular) to the direction the wave is moving.

Examples:

1.water waves

2.Electromagnetic waves

3.Waves on a spring or a stringTransverse Wave Animation

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Wave Typesb) Longitudinal Wave – The motion of the particles are in the same direction as the wave is moving. The wave pulsates back and forth in the direction it is moving.

Example:

sound wavesLongitudinal Wave Animation

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Example of what a speaker does to air molecules to produce sound

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Wave Typesc) Torsional Wave – Waves that cause the

medium to ‘twist’

Torsional Wave Animation

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Terminologies used in waves

1. Crests – High points on waves

2. Troughs – Low points on waves

Crest

Trough

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Cont’3. Amplitude – The distance from the midpoint of the wave to the crest or

trough.

Midpoint/Mean position

Amplitude

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4. Compression – The part of a longitudinal wave where the medium is being squeezed

together

Cont’

5. Rarefaction – The part of a longitudinal wave where the medium is being pulled

apart.Compression Rarefaction

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Cont’6. Frequency (ƒ) – How many of the same

part of the wave that pass a point in a certain time. How many pendulum cycles happen in

a given time.

Hertz (Hz) – Unit of frequency. One cycle per second (1/sec)

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Cont’

7. Period (T) – The time taken for one complete oscillation. Or

Time of one back-and-forth swing.

For waves, the time it takes for the same section of a wave to pass the same

point.

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WavesWavelength (λ) – The distance between two

identical points on a wave. Often said “From crest to crest, or trough to trough”.

Measured in meters.

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WavesWavelength (λ) – The distance between two

identical points on a wave. Often said “From crest to crest, or trough to trough”.

Measured in meters.

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• Measured in sec/event.

Period (T) – The time for one event to occur.

Waves

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# = number of events, cycles, etc.f = frequency (events per second, Hz)

t = elapsed time in seconds

Units:

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# = number of events, cycles, etc

T = Period (seconds per event, min per event, etc.)

t = elapsed time (usually in seconds)

Units:

22 T = period (seconds/event)

f = frequency, measured in hertz (Hz), events/second, or just 1/second

Units:

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WavesFrequency (ƒ) = 1 / period

Period (T) = 1 / frequency

The source of all waves is something that vibrates. The frequency of the vibrating

source and the frequency of the wave produced by the source is the same.

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Wave Speed Wave speed depends on what it is traveling

through. Sound travels about 340 m/s in air but about

4 times FASTER in water.

Its even faster through a solid substance. (More Dense = Faster Wave)

Would changing the frequency of a wave change its speed?

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v = wave speed (m/s)λ = wavelength (m)Units:

f = frequency (event/sec, Hz)

λ

Page 266 in your book

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Wave SpeedIf two waves pass the same point each second and have a wavelength of 3 meters then what

would the wave speed be?

v = 2 Hz • 3 m

6 m/s

v = ƒ • λ

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Wave SpeedAt a concert, the high notes and low notes get to you at the same time. The high notes have a shorter wavelength and higher frequency and the low notes have a longer wavelength

and smaller frequency.

The speed is the same.

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Wave Speed

Different Frequency, Same Speed (Because they are moving through the same medium)

HIGH PITCH

LOW PITCH

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End of Pre-Lab Notes

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InterferenceWaves can share the same space as they pass

through each other. When they do this, we say that the waves interfere with each other.

The waves can interfere with each other causing the effects of each wave on the medium to be

increased, decreased or possibly even eliminated.

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InterferenceConstructive Interference – When two of the same parts of two waves combine, increasing

or reinforcing themselves

If two identical wave crests hit each other, the wave will become twice as big.

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Constructive Interference

=Reinforcement

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InterferenceDestructive Interference – When two different parts of two waves collide,

decreasing or eliminating themselves

If a wave crest hits a wave trough, then the waves will either be lessened or completely

eliminated.

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Destructive Interference

=Cancellation - Total Destructive

Interference

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Example: Noise Cancelling Head Phones