Catching Good Vibrations

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Catching Good Vibrations

• Chapter 25
• Vibrations and waves

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I. Labs

• Types of pendulums simple and physical
• A. Tick Tock Lab
• Mass doesnt matter

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I. Labs

• A. Tick Tock Lab
• Mass doesnt matter
• Gravity does matter
• More gravity smaller period
• Less gravity longer period

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I. Labs

• B. Grandfather clock lab
• Length matters
• Long pendulum
• long period
• Short pendulum short period

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I. Labs

• B. Grandfather clock lab
• Length matters
• Long pendulum long period
• Short pendulum short period
• C. Compare the times on your data table with the
  person next to you. Discuss sources of error.

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II. Formula for period

• So what factors should be in the formula for
  period? What matters?
• Are they directly proportional or indirectly
  proportional?

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II. Formula for period

• T 2??L/g
• Does this make sense considering your data from
  your labs? Discuss with your neighbor and be
  ready to explain your answer

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• Is there a relationship between a pendulum and a
  wave?
• pendulum waves

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Simple harmonic motion

• Back and forth vibrating motion
• Periodic motion in which the restoring force is
  proportional to the displacement from the
  equilibrium
• Examples springs, pendulums
• Makes sine wave

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III. Wave description

• A. Parts of a wave

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III. Wave description

• B. Period
• The time it takes for a vibrating object to
  repeat its motion
• How long until vibration repeats
• Seconds/vibration or sec/cycle

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• C. Frequency
• The number of vibrations made per unit time
• How often a vibration occurs
• Vibrations/second or cycles/ sec
• hertz (Hz)

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• C. Frequency
• The number of vibrations made per unit time
• How often a vibration occurs
• Vibrations/second or cycles/ sec
• hertz (Hz)
• AM radio kHz kilohertz -- thousands
• FM radio mHz megahertz millions
• Microwave gHz gigahertz -- billions

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• D. frequency 1/period
• period 1/ frequency
• E. Source of waves is something that vibrates
• Frequency of source and wave is the same

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Concept quiz

• A pendulum has its mass doubled. How should the
  length change to keep the frequency the same?
• 1. Cut it in half
• 2. Double
• 3. Quadruple
• 4. None of these

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IV. Wave Speed

• A. Depends on the medium through which it travels
• Ex. Sound waves travel 330 m/s in air four times
  faster in water!!
• B. Medium, speed, wavelength and frequency are
  all related
• Velocity wavelength x frequency
• v ?f

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• Concept quiz
• Waves in a lake are 2.0 m in length and pass an
  anchored boat 0.5 s apart. What is the speed of
  the waves?
• 1. 2m/s
• 2. 1m/s
• 3. 4m/s
• 4. 8m/s

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• Concept quiz
• A boat at anchor is rocked by waves whose crests
  are 20 m apart and whose speed is 10 m/s. How
  often do these waves reach the boat?
• 1. Every second
• 2. Every 2 seconds
• 3. Every 2.5 seconds
• 4. Every 3 seconds

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• Concept quiz
• Who has the same speed as a wave with a
  wavelength of 1 m and a frequency of 1 Hz?
• 1. wavelength 2 m and f 2 Hz
• 2. wavelength 1 m and f 2 Hz
• 3. wavelength 2 m and f 1 Hz
• 4. none of these

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V. Wave Motion

• Play the scenario - when I say go
• Just for a moment line up around the perimeter of
  the room.
• The person closest to the door will start a wave
  by jumping up then down - that wave will move
  around the room, each person is a model for an
  atom.
• GO!

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Contemplate

• Which way did the atoms (you) move?
• Which way did the wave move? (the wave movement
  is called the wave propagation.)
• What was the angle between the two?
• Is there another word that could be used for
  this?
• The wave where the propagation of the wave is
  perpendicular to the movement of the particles is
  called a transverse wave.

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V. Wave Motion

• A. When energy is transferred from a vibrating
  source to a distant receiver there is no transfer
  of matter between the two points
• The disturbance rather than the medium moves
  along the spring

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VI. Types of waves

• A. Transverse
• Motion of the medium is at right angles to the
  direction in which a wave travels
• Examples light
• B. Longitudinal
• Particles move along the direction the wave
  rather than at right angles to it
• Example sound
• http//www.hamline.edu/arundquist/phys1140/animat
  ions/simplewaves.html

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Longitudinal Vs. transverse waves

• http//www.kettering.edu/drussell/Demos/waves/wav
  emotion.html
• What are the similarities and difference of the
  two types of waves weve looked at?
• Are the particles making a NET movement?
• Which direction relative to the propagation of
  the wave, are the particles moving in each case?

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VII. Interference

• A. Waves can exist at the same time and in the
  same space
• B. Form interference patterns where wave effects
  may be increased, decreased or cancel each other
• http//mysite.verizon.net/vzeoacw1/wave_interferen
  ce.html

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• C. Constructive interference
• Crest of one wave overlaps the crest of another
  and they add together
• D. Destructive Interference
• Crest of one wave overlaps the trough or another
• E. Out of phase out of step
• In phase in step

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VIII. Standing Waves

• A. Waves can be reflected
• If rope is attached to a wall and you shake the
  free end up and down, the wave is reflected back
  to you
• http//www.kettering.edu/7Edrussell/Demos/reflect
  /reflect.html

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VIII. Standing Waves

• A. Waves can be reflected
• If rope is attached to a wall and you shake the
  free end up and down, the wave is reflected back
  to you
• B. standing waves are caused by the incident wave
  and reflected wave
• C. Nodes are points that remain stationary
• D. Antinodes are the positions with the largest
  amplitude

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• E. Result of interference
• F. They are set up in string instruments and
  organs.
• http//www.hamline.edu/arundquist/phys1140/animat
  ions/standingwave.gif

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Concept quiz -- nodes

• If the spacing between nodes is 1.0 m, what is
  the wavelength of the wave?
• 0.1 m
• 0.5 m
• 1 m
• 2 m

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IX. Doppler Effect

• A. Apparent change in frequency due to the motion
  of the source
• Greater the speed of the source the greater the
  Doppler effect
• B. Example pitch of a siren is different coming
  towards you than going away.
• http//www.hamline.edu/arundquist/mathematica/gen
  eralphysics/waves/doppler.html

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• C. True for all types of waves
• Water , sound and light
• 1. Sound
• Changing pitch of a car horn as it passes
• Coming towards you it has a higher frequency
  which gives it a higher pitch
• Going away it has a lower frequency which gives
  it a lower pitch

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• C. True for all types of waves
• Water , sound and light
• 1. Sound
• Changing pitch of a car horn as it passes
• Coming towards you it has a higher frequency
  which gives it a higher pitch
• Going away it has a lower frequency which gives
  it a lower pitch
• The frequency changes not the speed of the wave

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• 2. Police use it for radar guns
• 3. Astronomers use it to see if light source is
  coming or going