Chapter 19 Vibrations and Waves

There are two ways to transmit information and/energy in our universe: Particle Motion and Wave Motion

Vibration - Wiggle in time Wiggle in space Wave - Light and Sound Both are vibrations of different kinds.

1. VIBRATION OF A PENDULUM Demo - Metronome Demo - Bowling ball pendulum Demo - Pendulum with extra mass Video - Three Bowling Balls Video - Swinging Examples

 T is the period, the time for one vibration.  l is the length of the pendulum.  g is the acceleration due to gravity.  Galileo discovered this.  Period (T ) is independent of the mass of the bob. Period of a Pendulum

Pendulum Uses: Timing Oil prospecting Walking When the oscillation is small, the motion is called simple harmonic motion and can be described by a simple sine curve.

2.WAVE DESCRIPTION Frequency ( f ) is the number of vibrations per unit of time made by the vibrating source. Units - cycles per second 1/s Hertz (Hz)

Picture of a Transverse Wave Crest Trough Wavelength A A - Amplitude

Distance between adjacent crests in a transverse wave Distance a wave travels during one vibration Units - meters or feet Wavelength ( )

  The period (T ) of a vibration is the time required to make one vibration.   The period (T ) of a wave is the time required to generate one wave.   It is also the time required for the wave to travel one wavelength.

Frequency Period

In symbolic form or

3.WAVE MOTION   Energy is transported by particles or waves.   A wave is a disturbance transmitted through a medium.   Exception: light does not require a medium.

Demo – Waves on a rope URL - Transverse wave A disturbance moves through the medium. Elements of the medium vibrate. Examples: ripples on water wheat waves

4.WAVE SPEED The average speed of anything is defined as

For a wave, if the distance traveled is a wavelength ( ), then the time to travel this distance is the period (T ). Then or Therefore Remember that

is true for all waves. Demo - Complete Bell Wave Machine Note: v is dictated by the medium. (must change medium to change v) f is dictated by the source. (must change the source to change f )

5.TRANSVERSE WAVES Video - Slinky Transverse Waves Examples: string musical instruments ripples on water electromagnetic waves

6.LONGITUDINAL WAVES Video - Slinky Longitudinal Waves Parameters Rarefactions are regions of low density. Compressions (condensations) are regions of high density.  is the distance between successive rarefactions or successive compressions.

Demo - Slinky Example: sound in air URL - Transverse & longitudinal waves

7.INTERFERENCE Video - Superposition of Waves Slide - Interference

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.

Interference is a characteristic of all waves.

Standing Waves   When two sets of waves of equal amplitude and wavelength pass through each other in opposite directions, it is possible to create an interference pattern that looks like a wave that is “standing still.” It is a changing interference pattern.  Demo - Rope and strobe  Demo - Mechanical overhead model

There is no vibration at a node. There is maximum vibration at an antinode. is twice the distance between successive nodes or successive antinotes.

 Video - Bell Wave Machine  Video - Drumhead Vibrations  Demo - Soda straw wave machine  Demo - Organ pipe and tuning fork  Another example: musical instruments

8.DOPPLER EFFECT Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves  Video - Doppler with Sound  Video - Doppler with Water  URL – Doppler Movie

When a source of waves and an observer of waves are getting closer together, the observer of the waves “sees” a frequency for the waves that is higher than the emitted frequency. When a source of waves and an observer of waves are getting farther apart, the observer of the waves “sees” a frequency for the waves that is lower than the emitted frequency.

  All waves exhibit the Doppler effect.   A particularly interesting example is used by astronomers to determine if light emitting objects (such as stars) are getting closer to us or farther away.   On average most stars are moving farther away, and their light spectra are “red shifted.”

Red Shift Lab Absorption Spectrum of Element X Star Absorption Spectrum of Element X Star is moving away from us. Red Shifted

  Radar bounced off a spinning planet can exhibit a Doppler effect and lead to a determination of the spin rate of the planet.   This was used to discover that Venus has a retrograde spin.   Of course police use the Doppler effect to catch speeding motorists.

9.BOW WAVES   Waves in front of moving object pile up.  Slide - Wave Barrier Slide  Slide - “Bow” Wave Slide   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.

10. 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.  Demo - Cone of Waves   There are two booms, one from the front of the flying object and one from the back.

 Demo – Crack whip  Video - FB-111 Sonic Boom  Video - FS14 Sonic Boom  Word Doc - Sonic Boom   The boom is not produced just when the flying object “breaks” through the sound barrier.

- faster than the speed of sound - slower than the speed of sound Subsonic Supersonic Mach Number = speed of sound speed of object

Chapter 19 Review Questions

Doubling the mass of a simple pendulum undergoing small oscillations does what to the period of the pendulum? (a) cuts it in half (b) increases it by the square of 2 (c) nothing (d) doubles it (c) nothing

What dictates the frequency of a sound wave? (a) wavelength (b) medium (c) source of the sound (d) speed (e) amplitude (c) source of the sound

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 (b) compressions

(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 (b) is less than what the source emits When you move away from a fixed source of sound, the frequency of the sound you hear

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

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