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Chapter 19

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Chapter 19

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  1. Chapter 19 Vibrations and Waves

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

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

  4. 1. VIBRATION OF A PENDULUM Demo - Metronome Demo - Bowling ball pendulum Video - Three Bowling Balls Video - Swinging Examples Demo - Pendulum with extra mass Time to swing depends on the length but not the mass of the pendulum.

  5. Period of a Pendulum • 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.

  6. 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.

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

  8. Picture of a Transverse Wave Crest l Wavelength A A - Amplitude Trough

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

  10. 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.

  11. Period Frequency

  12. In symbolic form or

  13. 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.

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

  15. 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

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

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

  18. 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 )

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

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

  21. Demo - Slinky Compressions Example: sound in air Rarefactions

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

  23. What determines the speed of a wave? (a) the frequency (b) the wavelength (c) the amplitude (d) the period (e) the medium of transmission

  24. 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

  25. 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

  26. 7. INTERFERENCE Video - Superposition of Waves Slide - Interference

  27. Interference Applet

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

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

  30. Interference is a characteristic of all waves.

  31. 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

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

  33. Video - Drumhead Vibrations • Demo - Organ pipe and tuning fork • Demo – Standing waves in sheet metal • Another example: musical instruments

  34. 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 Effect in Air • Video - Doppler Effect in a Ripple Tank • URL– Doppler Movie (htm) • Demo – Doppler Rocket

  35. When a source of waves and an observer of waves are getting closer together, the observer of the waves observes 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 observe a frequency for the waves that is lower than the emitted frequency.

  36. 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.”

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

  38. Police use the Doppler effect to catch speeding motorists. • 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.

  39. Planet Spinning Under Cloud Cover

  40. 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

  41. 9. BOW WAVES • Waves in front of moving object pile up. • Wave Barrier

  42. x x x x x x x

  43. “Bow” Wave

  44. x x x x x x x

  45. 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.

  46. 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.

  47. Demo – Crack whip • Video - FB-111 Sonic Boom • Video – F-14 Sonic Boom • URL – More Boom • Word Doc - Sonic Boom • The boom is not produced just when the flying object “breaks” through the sound barrier.