Mechanical Waves vs. Electromagnetic Waves Mechanical – require a medium to travel –ex: water,...
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Transcript of Mechanical Waves vs. Electromagnetic Waves Mechanical – require a medium to travel –ex: water,...
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Mechanical Waves vs. Electromagnetic Waves
• Mechanical – require a medium to travel– ex: water, sound, rope
• Electromagnetic – can travel through space– ex: light, microwaves, radio waves
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The Science of Sound
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Tuning forks create sound by
compressing the air around them.
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These compressions vibrate air molecules as they travel away
from the fork.
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Sound requires molecules to vibrate.
No molecules no sound.
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What’s wrong with this?
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So spaceships are silent in
space.
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Sound moves through air as
compressed air molecules.
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Compressions enter our ears, move things
around, and we hear.
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There are several parts to the human ear.
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Ear Lobe
Ear Hole
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The ear lobe serves as a reflector to bounce sound waves into the ear hole.
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The ear hole is how compressions enter the ear.
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The ear canal directs sound to the inner ear.
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The ear drum is a thin tissue. Compressions cause it to vibrate.
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The hammer is connected to the ear drum. When the drum moves, the hammer moves.
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When the hammer moves, it bumps the anvil.
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The anvil is connected to the stirrup so it vibrates with the anvil.
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The stirrup is then connected to the cochlea which turns vibrations into a signal for your brain.
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Does sound travel faster in liquids and solids or in gases?
In general, liquids and solids
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Can sound move in a vacuum?
No way…
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Sound travels faster in warm air than in
cooler air. On average, it moves at
about 343 m/s.
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That’s about as fast as a jet plane.
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Lets say you come across a dark, deep well and you
want to know how deep it is. What would you do to find
this out?
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With a thermometer (to know the temp.
of the air) and a stop watch you can
calculate how far your echo travels.
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Let’s say it’s an average
temperature day, and it takes 3
seconds for your echo to return. How
deep is the well?
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Speed of sound = 343 m/s
Time = 3 s
d = vt
So the sound travels for…
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1,029 m
Is that how deep the well is?
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It’s 1,029 m to the well and back! So the
distance to the well is 515 m!
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Two things we always notice about
sound waves are their amplitude and
their frequency.
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The amplitude of a sound wave, or the amount of energy it
has, is also known as its volume.
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We can say that sounds with high
volume are loud, and sounds with low
volume are quiet.
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Or that sounds with high volume have a
bigger amplitude than sounds with low
volume.
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LOUDNESS
Amplitude
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Too much amplitude
, or volume is
a dangerous
thing.
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We measure volume in an SI unit called the
decibel (dB).
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About 35 – 40 dB is an
average speaking volume.
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About 10 dB is a whisper, and anything over 80 dB, like a dance club or gun shot, can
cause hearing damage.
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The other thing we notice about sound waves is
their frequency. Musicians call frequency
pitch, or notes.
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When the frequency increases, the pitch, or
note, gets higher.
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When the frequency decreases, the pitch, or
note, gets lower.
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PITCH
Frequency
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An interesting thing happens to the frequency
of a sound when the source of the sound is
moving.
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What do you think is making this sound? Why
do you think the frequency is changing?
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The compressions in the direction the source is
moving bunch up.
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Meaning that the wavelength is less, so the
frequency, or pitch, is higher.
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The waves behind get more spread out, making
the frequency lower.
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Describe the motion of these cars.
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This is called the Doppler Effect.
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It looks like this.
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If the sound waves get more bunched up when the source moves faster, is it possible to make it so that they are all on top of
each other?
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Or maybe even to move so fast the they can’t
keep up?
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Yes. Jets that fly faster than 350 m/s travel
faster than the sound they produce. So we say
they ‘break the sound barrier’.
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It looks like this.
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The Doppler Effect
s
dsd vv
vvff
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Key Points
1. ALWAYS establish a coordinate system.
2. Make sure that the positive direction is from the source to the detector.
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A train moving toward a sound detector at 31.0 m/s blows a 305 Hz whistle. What frequency is detected on each of the following?
A stationary train
A train moving toward the first train at 21.0 m/s
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Now the train is moving away from the detector at 31.0 m/s and blows the same 305 Hz whistle. What frequency is now detected on each of the following?
A stationary train
A train moving away the first train at 21.0 m/s
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Example:• Jordan, a trumpet player, sounds C
above middle C (524 Hz) while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume the speed of sound is 343 m/s. (Start with a picture!)
• Answer: 564 Hz• Make sure you think logically about
what the magnitude of the frequency should be.