C19 Waves
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C19 Waves Presentation Transcript

  • 1. Chapter 19 Vibrations & Waves Mar 6, 2010 Physics 1 (Garcia) SJSU Part IV: Sound
  • 2. Vibrations
    • Vibrations are a common phenomenon.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Oscillating motion of a mass on a spring
  • 3. Period
    • Time required for a full oscillation (one round trip) is called the period of oscillation.
    • Pendulum that is about one meter long has a period of two seconds per oscillation.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Note: Measure the period of a pendulum in lab.
  • 4. Frequency
    • Frequency is the inverse of the period,
    • (Frequency) =
    • For example, for a period of 2 seconds per oscillation, the frequency is ½ oscillation per second or ½ Hertz.
    • 1 Hertz = 1 oscillation per second
    Mar 6, 2010 Physics 1 (Garcia) SJSU (Period) 1
  • 5. Amplitude
    • The distance from the rest position is the amplitude of oscillation.
    Amplitude
  • 6. Waves
    • Concept of vibrations extends into the phenomenon of wave motion.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Water waves Light waves Sound Radio String
  • 7. Transverse Waves
    • For transverse waves the wave’s amplitude is perpendicular to the wave’s motion.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Wave Motion Amplitude Amplitude
  • 8. Doing “The Wave”
    • Synchronized standing and sitting by a stadium’s crowd is an example of a transverse wave.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Video analysis indicates that it takes only a few dozen fans leaping to their feet with their arms up to trigger a wave. Once started, it usually rolls in a clockwise direction at a rate of about 40 feet per second, or about 20 seats per second. At any given time, the wave pulse is about 15 seats wide.
  • 9. Longitudinal Waves
    • For longitudinal waves, amplitude and wave motion are parallel.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Wave Motion Amplitude Amplitude A crowd can do a longitudinal wave by moving side-to-side instead of up-and-down
  • 10. Wavelength
    • Wavelength is distance between crests or between troughs of waves.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Longitudinal Transverse
  • 11. Wave Speed
    • The speed at which waves travel is called the wave speed .
    • Speed of sound = 330 m/s = 725 mi/hr
    • Speed of light = 300,000,000 m/s
    Mar 6, 2010 Physics 1 (Garcia) SJSU Water waves at the beach move a few miles per hour
  • 12. Tsunami Waves
    • Tsunamis are ordinary water waves, just like waves in your bathtub, but because they are typically generated by deep sea earthquakes they carry huge amounts of energy and momentum, traveling at almost 500 mph while in the deep ocean.
    Mar 6, 2010 Physics 1 (Garcia) SJSU The tsunami of 26 December 26 th 2004 was produced by an earthquake whose epicenter was located off the coast of Indonesia in the Indian Ocean. The death toll is estimated at over a quarter of a million persons. Animation by Vasily V. Titov
  • 13. Wave Relations
    • Wave speed, wavelength & frequency related.
    • (Wave speed) = (Wavelength) x (Frequency)
    • (Wave length) =
    • (Frequency) =
    Mar 6, 2010 Physics 1 (Garcia) SJSU (Wave speed) (Frequency) (Wave speed) (Wavelength)
  • 14. C heck Yourself
    • What is the wave length?
    • What is the wave speed?
    • What is the wave’s frequency?
    • What is the wave’s period?
    Mar 6, 2010 Physics 1 (Garcia) SJSU 6 m/s 2 m
  • 15. Demo : Hearing Sound
    • Range of human hearing is roughly 20 Hertz to 20,000 Hertz.
    • Wave speed for sound is 330 m/s
    • Wavelength of 20 Hertz is 16 m (about 50 ft)
    • Wavelength of 20,000 Hz is 1.6 cm (½ inch)
    Mar 6, 2010 Physics 1 (Garcia) SJSU
  • 16. Hearing in Animals Frequency range varies widely, depending on natural adaptation using sound to communicate, locate food, avoid predators, etc.
  • 17. Constructive Interference
    • Two waves in phase add together, which is called constructive interference.
    Mar 6, 2010 Physics 1 (Garcia) SJSU
  • 18. Destructive Interference
    • Two waves out of phase cancel each other out, which is destructive interference .
    Mar 6, 2010 Physics 1 (Garcia) SJSU
  • 19. In & Out of Phase
  • 20. Demo : In & Out of Phase
    • Pair of speakers constructively interfere when they are in phase (oscillating together).
    • When out of phase (reverse wires on one of the speakers) then they destructively interfere.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Out of Phase
  • 21. Noise-Canceling Headphones
    • Noise-canceling headphones use a microphone that listens for noise and a speaker that produces the same noise but out of phase (cancellation by destructive interference)
    External Noise Canceling Sound
  • 22. Demo : Speaker Baffle
    • Why are speakers mounted behind a baffle and inside an enclosure?
    Mar 6, 2010 Physics 1 (Garcia) SJSU
  • 23. Standing Waves
    • When a wave interferes with its reflection, this may create a standing wave.
  • 24. Standing Waves and Phase Mar 6, 2010 Physics 1 (Garcia) SJSU Standing waves are another example of constructive and destructive interference.
  • 25. Doppler Effect
    • Sound coming from a moving object has a different wavelength and frequency than if it were stationary.
    • If moving towards you, wavelength shorter and frequency higher.
    • If moving away, wavelength longer and frequency lower.
    Mar 6, 2010 Physics 1 (Garcia) SJSU
  • 26. Demo : Doppler Shift
    • Hear frequency as higher when buzzer is moving towards you and hear it as lower when moving away from you.
    Mar 6, 2010 Physics 1 (Garcia) SJSU Higher Frequency Lower Frequency Longer Wavelength Shorter Wavelength