Vibrations and Waves

Vibrations and Waves

• Oar in Water

• Wings of a Bee

• Electrons in an Light Bulb

• Water Waves

• Sound Waves

• Light Waves

“Wiggles in Time” “Wiggles in Space”

Vibrations and Waves

• Waves transmit energy and information.

• Sound and Light are both waves.

Simple Harmonic Motion...

• …is to-and-fro vibratory motion.

• ...results in sine curves.

• Examples: – metronome– mass on a spring– pendulum

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

Wave Description

• Period - the time required for one vibration

• measured in seconds

• Frequency - number of vibrations per unit time

• measured in Hertz

Bowling Ball Example

Wave Description

Bowling Ball Example

FrequencyPeriod

1

PeriodFrequency

1

Question:

If you double the frequency of a vibrating object, what happens to the period?

a) the period doublesb) the period stays the samec) the period is cut in half d) not enough information is given to answer this question.

What is the frequency in vibrations per second of a 60-Hz wave?

What is its period?

What is the frequency in vibrations per second of a 60-Hz wave?

Answer: 60 cycles per second

What is its period?

Answer: 1/60 second

Wave Motion

• medium - the stuff that carries the wave

Waves Mediumwater 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

• It depends only on the type of medium.

• Wave Speed = Frequency Wavelength

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?

What is its period?

What is its wavelength?

What is its wave speed?

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 HzWhat is its period?

Answer: 1/3 secondWhat is its wavelength?

Answer: 2 mWhat is its wave speed?

Answer: 6 m/s

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

INTERFERENCE

• Constructive or destructive interference results when waves add.

• Standing Waves - wave pattern produced from interfering waves– Examples

• Vibrating Strings

• Organ Pipe

Waves

Constructive Interference occurs when waves are in phase, that is when crests are superimposed and troughs are superimposed.

Destructive interference occurs when waves are out of phase, that is when crests are superimposed with troughs.

Standing Waves

Standing Waves

Some Applets

http://www.kettering.edu/~drussell/Demos/superposition/superposition.html

http://www.ngsir.netfirms.com/englishhtm/TwaveStatA.htm

http://id.mind.net/~zona/mstm/physics/waves/standingWaves/understandingSWDia1/UnderstandingSWDia1.html

http://www.kettering.edu/~drussell/Demos/waves/wavemotion.html

http://www.ngsir.netfirms.com/englishhtm/StatWave.htm

DOPPLER EFFECT

• the change in wavelength due to motion of the source

• "Wheeeeeeeeeeee…….Oooooooooooooo”

• Example:– moving cars and trains

DOPPLER EFFECT

BOW WAVES

• Waves in front of moving object pile up.

• The familiar bow wave generated by a speedboat knifing through the water is a non-periodic wave produced by the overlapping of many periodic circular waves. It has a constant shape.

SHOCK WAVES• Just as circular waves move out from a swimming bug, spherical

waves move out from a flying object. If the object flies faster than the waves, the result is a cone-shaped shock wave.

• There are two booms, one from the front of the flying object and one from the back.

SHOCK WAVES

Class Problem

Dipping a finger in water faster and faster causes the wavelength of the spreading waves toa) increase

b) decrease

c) stay the same

d) not enough information is given

Class Problem

• In the standing wave shown, what is its amplitude? What is its wavelength? How many nodes are there?

Class Problem

• In the standing wave shown, what is its amplitude? What is its wavelength? How many nodes are there?

• The amplitude of the wave is 10 centimeters; the wavelength is 1 meter; and there are 6 nodes.

What determines the speed of a wave?

(a) the frequency

(b) the wavelength

(c) the amplitude

(d) the period

(e) the medium of transmission

A skipper on a boat notices wave crests passing his anchor chain every 5 seconds. If the wave crests are 15 m apart, what is the speed of the water

waves in m/s?

(a) 5(b) 15(c) 75(d) 10(e) 3

For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased

are called

(a) rarefactions

(b) compressions

(c) density holes

Class Problem

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

What dictates the frequency of a sound wave?

(a) wavelength

(b) medium

(c) source of the sound

(d) speed

(e) amplitude

When you move away from a fixed source of sound, the frequency of the sound you hear

(a) is greater than what the source emits

(b) is less than what the source emits

(c) is the same as what the source emits

Sonic booms from a plane are produced

(a) because the plane breaks through the sound barrier

(b) when the plane reaches the speed of sound

(c) by the plane traveling faster than the speed of sound

(d) by the plane traveling slower than the speed of sound