wave lecture (narrows bridge: wave lecture intro: ( bill nye: waves
TRANSCRIPT
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Wave Lecture(narrows bridge: http://www.youtube.com/watch?
v=xox9BVSu7Ok)wave lecture intro: (http://www.youtube.com/watch?
v=jAXx0018QCc)bill nye: waves
(http://www.youtube.com/watch?v=OGyRe_SGnck)
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1. Wave properties
A. waves are the transfer of kinetic energy through a medium or vacuum:
-sound though air,
-light through space: vacuum
-heat through a metal pan: solid
B. when waves travel through matter:
1. it transfers kinetic energy
2. but medium/matter remains in place:
-grass blades waving in the wind,
-sound through walls
- ocean waves
3. the energy in motion carries information: sound, pictures and numbers
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matter remains in place: net circular motion
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C. waves have a repeating cycle (back-and- forth), called an oscillator,
-like a person on a swing (pendulum) or
-a vibrating string on a guitar
1. repeating motion is also called harmonic:
heartbeat,
solar seasons,
singing in
harmony
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Harmonic wave motion
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2. one full revolution is called a cycle,
known as one unit of harmonic motion
3. waves move around a point of equilibrium: where the system is at “rest”, with a net force of zero
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D. energy/inertia pulls the wave away from equilibrium and the restoring force brings the wave back to the point of equilibrium:
= restoring force
= equilibrium
Demo Newton’s cradle:Newton’s first law
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E. wave part/labels
1. crest: top of the wave
2. trough: bottom of the wave
3. equilibrium: half way between crest and trough
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F. wavelength: distance from crest to crest,
(or trough to trough)
- Greek symbol lambda, = upside down y:
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G. amplitude: size of a wave (cycle)
1. wave height from point of equilibrium to top of crest (or half wave height from
trough to crest)
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2. amplitude will reduce (dampen) toward equilibrium with less wave energy
-direct relationship
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H. frequency: how often vibrations (wave cycles) occur
1. number waves per time:
-usually the number of wave crests per second
2. unit is the Hertz (Hz) = # waves
second
low frequency
high frequency
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I. radio waves:
-AM radio: alters amplitude of waves (height)
-FM radio: alters frequency of waves (how often)
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J. period: amount of time for a cycle: crest to crest (how much time for a class period?)
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K. frequency and period are inverse relationships:
period = 1
frequency
frequency = 1
period
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L. wave speed
1. within same medium = same speed
2. different densities of matter cause different wave speeds:
more dense =
more inertia =
slower wave speed
- an inverse relationship
3. greater the elasticity of the medium (ability to bounce back to original position), the faster the wave speed
- ie: steel faster than air: tap on a steel wall vs talking through air
- direct relationship
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4. wave speed =
-distance/time or
-frequency x wavelength =
(# waves/sec x distance crest to crest)
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2. Wave types
A. transverse waves (sine waves)
1. medium motion (oscillation) is perpendicular to the wave motion (up/down)
- as a water wave moves toward the shore, the crests/troughs move up
and down
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-examples of transverse waves:
light, heat, ocean/water, earthquake, Tacoma Narrows bridge
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B. longitudinal waves
1. medium motion in same direction (parallel) as wave motion (oscillation)
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2. waves of alternating regions of matter crowded together (compressions)
and matter spread apart: rarefactions
3. sound waves are longitudinal waves
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3. Wave interactions
- when energy waves move they are influenced by the boundaries they collide with and influence other waves they run into
A. reflection (as in a mirror image)
- wave/energy is not absorbed but bounced, how much depends the surfaces’ degree of absorption
- example: mirrors, barrier reefs, sound walls
- incoming wave angle (incident wave) =
outgoing wave angle (reflected)
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reflected wave: incident = reflected
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reflected waves: wavelength and frequency remain unchanged, usually (if energy is not absorbed)
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B. refraction
- bending of wave due to a change in speed- due to changing one medium to another
medium, thus changing density of the medium
-pencil in water:
- how would a pencil in oil look?
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light refraction from air to glass mediums (what album cover?):
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C. diffraction
-waves bending around or through an obstacle: sound traveling around a corner
-waves passing through a hole: changes to circular waves
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diffraction waves: circular waves produced: explains why sound can be heard through small opening.
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D. absorption
-waves can get smaller as they transfer energy into the medium they travel through:
ie: light passing through sunglasses, sound passing through sound boards in a recording studio
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E. wave to wave interference
- when two or more waves collide at the same point in space and time
1. constructive
a. wave crests meet and combine
b. form a single wave
c. amplitude is increased: additive of two original waves
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d. resonance
- when the natural frequency of an object matches the input wave frequency
- causes a dramatic increase of energy and thus an increase in amplitude of the generated wave:
-musical instruments
-Tacoma narrows bridge
-Calvary troops across marching across a footbridge in Manchester,England (1831)
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constructive interference
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2. destructive
a.when the crest of one wave meets the trough of a second wave
b. forms a single wave
c. amplitude is decreased: energy is subtracted
d. ie: anti-noise technology for earphones
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and so, the sun’s waves set yet again at the ocean waves, groovy!
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