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
fT 12
Wave Properties
• Period –
• Wavelength and Wave number –
fT 12
2
http://seagrant.uaf.edu/marine-ed/curriculum/images/stories/grade7/wave.jpg
Wave Properties
• Period –• Wavelength and Wave number –
fT 12
2
http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/Images_specific/wave_def.gif
Wave Properties
• Period –
• Wavelength and Wave number –
• Wave speed –
fT 12
2
http://eqseis.geosc.psu.edu/~cammon/HTML/Classes/IntroQuakes/Notes/Images_specific/wave_def.gif
fTt
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 –
fT 12
2
fTt
xv
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