Vibrations and Waves

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Vibrations and Waves

• Chapter 25

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The source of all waves is something that
vibrates !
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Simple Harmonic Motion (SHM)

• Vibration a movement (wiggle) in time
• Cycle on complete back-and-forth trip
• Simple Harmonic Motion the back-and- forth
  vibrations (oscillation)
• Makes a straight line on stationary surface
• Makes a sine curve on moving paper
• Wave a movement in space and time, a
  disturbance in a medium (material)

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As a wave moves through a medium it has a pulse.
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Energy (not matter) is transferred by a wave
from a vibrating source to a distant receiver.

• It travels outward in concentric circles
• by the disturbance of a medium.

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Types of Waves

• Transverse waves motion of the medium is at
  right angles (-) to the direction in which the
  wave travels.
• Up and down
• Crest and troughs

• Longitudinal waves motion of the medium is
  along the same direction (ll) as wave travels.
• Back and forth
• Compression and expansion

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Slinkies as waves
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Examples of Waves

• Mechanical - have a medium
• Surface (Water/Seismic) - transverse
  longitudinal
• Sound - longitudinal
• Electromagnetic (EM) no Medium
• They can travel through space (vacuum)!
• Light/Radio/Television - Transverse

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Surface Waves

• The medium undergoes a circular motion because of
  the combination of transverse and longitudinal
  motion.

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Ocean Wave Types

• Some types of Ocean Waves are
• Wind Generated
• Tides
• Tsunamis

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Describing Waves

• Crest the high points
• Troughs the low points
• Midpoint home or rest position
• Height distance from crest to trough
• Amplitude displacement from the midpoint to the
  crest (1/2 height)
• Wavelength (?) distance between successive
  identical parts of the wave

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Wave Energy is proportional to Amplitude

• Therefore,
• the higher the wave,
• the more destructive!

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Measuring Waves

• Period (T) The time it takes for a vibrating
  object to repeat its motion (SI second (s))
• Frequency (f) how often a vibration occurs or
  the number of vibrations made per unit of time
  (SI hertz (Hz))
• They are reciprocals of each other
• T 1 / f f 1 / T

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Wave Speed

• Depends on type of medium and temperature
• Ex Sound travels faster in water
• Ex Sound travels faster in warm air
• Wave speed wavelength frequency
• v ? f
• SI meters/second (m/s)

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Wave Speeds/Frequencies

• Speed of Sound (in air) 340 m/s
• Speed of Light (EM waves) 3x108 m/s
• Sound is heard between 20-20,000 Hz
• FM radio frequencies megahertz (MHz)
• AM radio frequencies kilohertz (kHz)

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Wave Interactions

• Reflection bouncing back at a surface/boundary
  (ex light)
• Refraction bending at a medium change, due to
  change in wave speed (ex light)
• Diffraction bending as it passes an edge or
  opening (ex sound)
• Interference combining waves

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Interference

• In/out phase has to do with the alignment
  (p.380)
• Constructive reinforcement, additive, crests
  combine to create larger wave
• Destructive cancellation, subtractive, the
  crest of one wave overlaps the trough of another

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Constructive Interference
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Destructive Interference

• Since these waves are equal and opposite, they
  obliterate one another.

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Interference creates Standing Waves

• When a wave appears to stand still in a limited
  space (used in musical instruments)
• The wave reflects back and forth from one end to
  the other
• Nodes (B point of
• minimum movement)
• Antinodes (A point of
• maximum movement)

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Doppler Effect

• A change in the apparent frequency (pitch) of
  sound due to the motion of the source of the
  receiver
• If a source approaches, the perceived frequency
  will be higher
• If a source recedes, the perceived frequency will
  be lower

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Alphabet Soup

• T period (seconds, s)
• f frequency (hertz, Hz) (1 Hz 1/s)
• v wave speed (m/s)
• ? wavelength (m)

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After Spring Break we will study Sound and Light!
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Created by Bugglady and Kathryn Johnson

• February 2006