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Vibrations & Waves

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  1. 19 & 20 Vibrations & Waves

  2. Sound in a Vacuum • Sound is the propagation of vibrations through a material medium—a solid, liquid, or gas. • If there is no medium to vibrate, then no sound is possible. • Sound cannot travel in a vacuum.

  3. Vibrations and Waves • Oar in Water • Wings of a Bee • Electrons in an Light Bulb “Wiggles in Time” “Wiggles in Space” • Water Waves • Sound Waves • Light Waves

  4. Vibrations and Waves • Waves transmit energy and information. • Sound and Light are both waves.

  5. If you drop two balls of different mass they accelerate at g. • Let them slide without friction down the same incline, and they slide together at the same fraction of g. • Tie them to strings of the same length so they are pendulums, and they swing to and fro in unison. • In all cases, the motions are independent of mass

  6. Pendulums & Galileo • The period does not depend on the amount of mass. • The period does depend on the length of the pendulum.

  7. 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. Note: Measure the period of a pendulum in lab. Physics 1 (Garcia) SJSU

  8. 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 1 (Period) Physics 1 (Garcia) SJSU

  9. Amplitude The distance from the rest position is the amplitude of oscillation. Amplitude

  10. A sine curve is a pictorial representation of a wave. • Amplitude • the distance from the midpoint to the crest (or trough) of the wave. • So the amplitude equals the maximum displacement from equilibrium. Like a water wave, a sine wave has crests, troughs and amplitude

  11. Waves Concept of vibrations extends into the phenomenon of wave motion. String Sound Water waves Radio Light waves Physics 1 (Garcia) SJSU

  12. The to-and-fro vibratory motion (often called oscillatory motion) of a swinging pendulum in a small arc is called simple harmonic motion. Wave Description

  13. Wave Motion • medium - the stuff that carries the wave Waves Medium water water waves rope waves on a rope people stadium waves sound air light space (vacuum)

  14. water wave • barring obstacles the surface of the water will have been disturbed, but the water itself will have gone nowhere. • A leaf on the surface will bob up and down as the waves pass, but will end up where it started. a stone dropped into a quiet pond has waves travel outward in expanding circles

  15. Wave Motion • When energy is transferred by a wave from a vibrating source to a distant receiver, there is no transfer of matter between the two points • The energy transferred from a vibrating source to a receiver is carried by a disturbance in a medium, not by matter moving from one place to another within the medium

  16. Wave Speed... • the speed with which waves pass by a particular point • e.g. the speed of a surfer • It depends only on the type of medium. • Wave Speed = Frequency  Wavelength Waves on a Rope Table in Notes – Appearance, Node, Antinodes, Wavelength, Frequency

  17. The frequency of the vibrating source and the frequency of the wave it produces are the same. • A station at 101.7 MHz FM -a frequency of 101,700,000 hertz. • The source of all waves is something that vibrates. Electrons in the transmitting antenna vibrate 940,000 times each second and produce 940-kHz radio waves.

  18. Wave Motion

  19. Wave Speed... • the speed with which waves pass by a particular point • e.g. the speed of a surfer • It depends only on the type of medium. • Wave Speed = Frequency  Wavelength Waves on a Rope Table in Notes – Appearance, Node, Antinodes, Wavelength, Frequency

  20. Wave Speed Problem The water waves below are traveling with a speed of 2 m/s and splashing periodically against the Wilbert's perch. Each adjacent crest is 4 meters apart and splashes Wilbert’s feet upon reaching his perch. How much time passes between each successive drenching?

  21. Transverse Waves For transverse waves the wave’s amplitude is perpendicular to the wave’s motion. Amplitude Wave Motion Amplitude Physics 1 (Garcia) SJSU

  22. Wavelength Wavelength is distance between crests or between troughs of waves. Longitudinal Transverse Physics 1 (Garcia) SJSU

  23. Transverse Waves • side to side vibration in a direction perpendicular to the wave's motion • Examples: • water waves • waves on a rope • string musical instruments

  24. Transverse Waves

  25. Longitudinal Waves • back and forth vibration in a direction parallel to the wave's motion • Examples: • slinky waves • sounds waves (vortex box)

  26. Longitudinal Waves

  27. Doing “The Wave” Synchronized standing and sitting by a stadium’s crowd is an example of a transverse wave. 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. Physics 1 (Garcia) SJSU

  28. Waves generated by an earthquake. P waves are longitudinal and travel through both molten and solid materials. • S waves are transverse and travel only through solid materials. • Reflections and refractions of the waves provide information about the Earth’s interior.

  29. Longitudinal Waves For longitudinal waves, amplitude and wave motion are parallel. Wave Motion Amplitude Amplitude A crowd can do a longitudinal wave by moving side-to-side instead of up-and-down Physics 1 (Garcia) SJSU

  30. Transverse vs LongitudinalDemonstration

  31. 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 Water waves at the beach move a few miles per hour Physics 1 (Garcia) SJSU

  32. 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. The tsunami of 26 December 26th 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 Physics 1 (Garcia) SJSU

  33. Wave Relations Wave speed, wavelength & frequency related. (Wave speed) = (Wavelength) x (Frequency) (Wave length) = (Frequency) = (Wave speed) (Frequency) (Wave speed) (Wavelength) Physics 1 (Garcia) SJSU

  34. Ruben’s Flame Demo

  35. If a water wave oscillated up and down three times each second and the distance between wave crest is 2 m, what is its frequency? Answer: 3 Hz What is its period? Answer: 1/3 second What is its wavelength? Answer: 2 m What is its wave speed? Answer: 6 m/s

  36. * Example Test Question: If you double the frequency of a vibrating object, what happens to the period? a) the period doubles b) the period stays the same c) the period is cut in half d) not enough information is given to answer this question.

  37. Example Question * Changing which of the following affects the period of a pendulum? • a) mass • b) amplitude • c) length • d) angle

  38. What is the frequency in vibrations per second of a 60-Hz wave? Answer: 60 cycles per second What is its period? Answer: 1/60 second

  39. 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) Physics 1 (Garcia) SJSU

  40. Hearing in Animals Frequency range varies widely, depending on natural adaptation using sound to communicate, locate food, avoid predators, etc.

  41. Wavelength Transverse Wave Crest Amplitude Baseline Trough

  42. INTERFERENCE • Constructive or destructive interference results when waves add. • Standing Waves - wave pattern produced from interfering waves • Examples • Vibrating Strings in Lab • Organ Pipe in Lab • Bell Wave Machine in Class

  43. Interference • Interference Pattern – within a pattern, wave effects may be increased, decreased, or neutralized • Constructive Interference (reinforcement) – the crest of one wave overlaps the crest of another, their individual effects add together producing an increased amplitude • Destructive Interference (cancellation) – the crest of one wave overlaps the trough of another, their individual effects are reduced • When waves produce areas of zero amplitude, they are “out of phase” • When the crest of one wave overlaps the crest of another, they are “in phase”

  44. constructive interference. • destructive interference • When the crest of one wave overlaps the trough of another, their individual effects are reduced. The high part of one wave simply fills in the low part of another. when the crest of one wave overlaps the crest of another, their individual effects add together to produce a wave of increased amplitude.

  45. Example Wave 2 ½ meters 20 cm Amplitude = 10 cm Wavelength = 1 m Number of Nodes = 6

  46. 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

  47. 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. Today you will create such patterns on a vibrating string.

  48. Parameters of a Standing Wave 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