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Vibrations

Vibrations. A vibration is a repeated back and forth motion. Examples are a child on a swing, a pendulum, a spring with an attached weight, a drum being struck, a guitar string being strummed, and alternating current. Waves. A wave is a vibration traveling through time.

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Vibrations

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  1. Vibrations • A vibration is a repeated back and forth motion. • Examples are a child on a swing, a pendulum, a spring with an attached weight, a drum being struck, a guitar string being strummed, and alternating current.

  2. Waves • A wave is a vibration traveling through time. • A wave carries energy between two points. • The medium is the material that the wave travels through. • The medium does not travel with the wave, the energy just passes through.

  3. Examples • Ocean waves carry energy through vast distances. Water particles move in small ellipses.

  4. Examples • Pendulum swinging back and forth. Why does it swing?

  5. Examples • Period of a pendulum depends only on its length. • T = 2π(L/g)1/2 • Calculate the period of a 0.8 m long pendulum. • Calculate the length of a pendulum with a period of 1.4 seconds.

  6. Pendulum waves • http://www.youtube.com/watch?feature=player_embedded&v=yVkdfJ9PkRQ

  7. Wave types • Mechanical waves require a medium in order to travel. Include pendulums, ocean waves, sound. • Electromagnetic waves do not require a medium to travel. Include all of the electromagnetic spectrum.

  8. Wave types • Transverse waves have particles that move perpendicular to the direction of wave motion.

  9. Wave types • Longitudinal waves have particles that move in the same direction of wave motion.

  10. Examples • Longitudinal: slinky waves and sound. • Transverse: Ocean waves, pendulums, light. • Mechanical: Ocean waves, pendulums, slinky waves, sound. • Electromagnetic: Only the electromagnetic spectrum, which includes light.

  11. Wave description • Crest is top of wave. Trough is bottom of wave. • Cycle is one complete swing back and forth. • Wave height is vertical distance between crest and trough.

  12. Wave description • Wavelength is horizontal distance from crest to crest or trough to trough • Amplitude is maximum height of a crest or depth of a trough relative to equilibrium level

  13. Wave description • Frequency, f, is number of crests that pass a given point per second, or the cycles per second. • Period, T, is time for one full wave cycle to pass • T = 1/f f = 1/T

  14. Wave description • Wave speed is how fast the wave passes a given point. • If a 3 m long wave passes a point 4 times in a second, what is it’s speed? • If wavelength = l, v = lf • Calculate speed of a wave that has a wavelength of 10m and a frequency of 3Hz.

  15. Wave animations • http://www.passmyexams.co.uk/GCSE/physics/basic-waves-theory.html

  16. Standing waves • Standing waves occur when two waves of identical wavelength and amplitude pass each other in opposite directions.

  17. Harmonic frequencies • http://www.youtube.com/watch?v=-n1d1rycvj4&feature=endscreen&NR=1

  18. Doppler effect • Doppler effect is an apparent change in frequency of a wave as it moves toward or away from an observer. • Since v = lf, and the velocity of sound does not change, what happens to f if the source moves toward you? • If it moves away from you?

  19. Sound • Sound is a longitudinal, mechanical wave created by a vibration. • Energy transfers from molecule to molecule as the wave passes. • Sound will not travel in a vacuum. • Sound travels at different speeds in different media. • Slow in air, fast in solids. Why?

  20. Intensity and loudness • Intensity is proportional to the square of the amplitude of a sound wave. • How does the intensity of a wave with 3 times the amplitude compare to the original wave? • Loudness is qualitative. What I think is loud, you may think is rather quiet.

  21. Resonance • Why do you pump your legs when on a swing? • Do you do this when going in the same direction or the opposite? • This is resonance. • Vibration at natural frequency creates resonance and can lead to failure of object.

  22. Resonance • http://www.youtube.com/watch?v=17tqXgvCN0E • http://www.youtube.com/watch?v=IqK2r5bPFTM

  23. Interference Destructive Constructive

  24. Interference and Beats

  25. Beats http://www.youtube.com/watch?v=3M5sYWjwfeg

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