PHYS16 – Lecture 37

Ch. 16 Wave Motion

“Interference is much clearer in HD”

Oscillations pre-question

• Is a bouncing ball an example of simple harmonic motion?

A) YesB) No

Oscillations pre-question

• Two kids are swinging on two swings of the same height – one kid is a little chubbier than the other. Neglecting frictional forces, which kid completes a back and forth swing in the fastest time?

A) The chubby kidB) The skinny kidC) The kid who pushes off the ground the bestD) Both complete in the same time

Oscillations pre-question

• A mass on a spring is driven at a driving frequency (red curve) and the resulting position vs. time of the mass is given (blue curve). Which case is closest to resonance?

Ch. 16 Wave Motion

• Mechanical Waves– Transverse vs. Longitudinal– Properties

• Example: Wave on a String– How the wave propagates– Speed– Reflection and Transmission

• Superposition and Standing Waves

Waves pre-question

PhET website

Waves pre-question

PhET website

0.0133

Mechanical Waves

Mechanical Waves

• A propagating oscillation that moves through a medium

Transverse – water waves, disturbance on a string

Longitudinal – sound waves, slinky or spring

Mechanical Waves

• Waves use at least 1-3 space dimensions and time…– Ex. – Water waves propagate in x, move “up and down” in

y, and change position with time– Ex. – Sound waves propagate in x, move “up and down” in

x, and change position with time

Wave Properties

• Period –

Thighperiod

mediumperiod

lowperiod

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Wave Properties

• Period –

• Wavelength and Wave number –

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2

http://seagrant.uaf.edu/marine-ed/curriculum/images/stories/grade7/wave.jpg

Wave Properties

• Period –• Wavelength and Wave number –

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http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/Images_specific/wave_def.gif

Wave Properties

• Period –

• Wavelength and Wave number –

• Wave speed –

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http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/Images_specific/wave_def.gif

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xv

Ripple Tank Simulation

Example: Speaker

PhET website

Speed =Period =Wave number = Wavelength =

330 m/s0.0038 s5.0m-1

1.25 m

Example: Sand on a vibrating plate

• Sand is placed on the vibrating plate below. Which apparatus is vibrating the plate with the highest wave number?

Wave Function or Waveform

)sin(),(22 ,

0

xtAtxyvfvvxt

Plane Wave

Higher Dimension Wave

)sin()(),( 0 xtxAtxy

Wave on a String

Wave on a String

• You can generate a wave on a string by pulsing sting with your hand

• Speed of pulse/wave is related to tension in string and mass density of string Tv

PhET website

Propagation of Pulse• If you advance the movie one frame point A will move…

(higher, lower, right, left)?• If generate new pulse but more quickly than first then it will

be… (same, wider, narrower)?• If generate new pulse but move hand higher than first then it

will be… (same, taller, shorter)?• If generate new pulse with rope tightened then pulse will be…

(same, faster, slower)?

PhET website

Wave on String Simulation

PhET website

Example Question

PhET website

Superposition, Interference, and Standing Waves

Main Points – Wave Properties

• Mechanical Wave – a propagating oscillation that moves through a medium Tv

• Period –

• Wavelength and Wave number –

• Wave speed –

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2

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xv