1 waves and vibrations 2 common wave characteristics: waves come in many types: water, sound, radio,...
TRANSCRIPT
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Waves and Vibrations
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Common Wave Characteristics:
Waves come in many types: water, sound, radio, light, etc.
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Wave Parts:
T = 1/f
Frequency: the number of waves that pass a given point in a unit of time - given in Hz - symbol f
Period: the time it takes for 1 full wave to pass a given point- given in seconds - symbol T
Sine wave
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Wave Parts:
Crest: top of wave
Trough: bottom of wave
Wavelength: distance between any two similar successive points on wave.
Amplitude: height of wave from midpoint.
Sine wave
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Wave Motion
Waves transport energy, not particles.
Imagine a fishing bobber floating on top of the water. As water waves pass, the bobber may move up and down, but it stays in the same
horizontal location.
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Wave SpeedGeneric wave speed formula that holds true for all
types of waves:
wave speed = wavelength x frequency
v = λ fλ = Greek letter “lambda”, represents wavelength.
Ex: What is the wave speed of a water wave moving with a frequency of 15 hz and wavelength of 5 m?
V= (5 m) x (15 hz) =
75 m/s
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Two Types of Waves
Transverse Wave: Wave particles vibrate perpendicular to the direction of wave motion.
Ex: water waves, waves in a rope...
http://www.colorado.edu/physics/phet/simulations/stringwave/stringWave.swf
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Longitudinal Wave: Wave particles vibrate along the direction of the wave.
Ex: sound waves
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Two Other Types of WavesMechanical Waves: waves that require a medium to
travel through.
Medium: the substance a wave travels through.
Water waves are mechanical waves. Without water, no wave!
Electromagnetic waves: waves that don’t require a medium to travel.
Ex: any type of electromagnetic radiation.
Visible light, ultraviolet, and infrared rays from the sun reach earth through the vacuum of space, no medium needed.
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Notice the wide variety of waves in the EM spectrum!
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Waves Quiz1. What are the differences/similarities between:a.) Mechanical vs. Electromagnetic Waves
b.) Transverse vs. Longitudinal waves
c.) Draw a Transverse wave (at least 2 cycles) and label the following:
crest, trough, wavelength, & amplitude
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Doppler Effect
A stationary source of waves would produce concentric wave circles:
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A moving source would create waves that are centered closer to the direction of movement.
The Doppler effect is a shift in frequency due to
a moving source or receiver.
It is often observed as a car horn, or siren passes
you.
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Police use the doppler effect to detect speeders. When the waves bounce off the oncoming car,
its frequency is shifted. This is used to calculate a speed.
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Astronomers use the doppler shift of light from distant galaxies to learn about their motions.
Objects that are moving towards the observer are said to be blue shifted.
blue = higher frequency.
Objects that are receding are said to be red shifted.
red = lower frequency.
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Shock Waves:As a source of waves nears the actual speed of the waves it produces, those waves “pile up”.
The “V” shape of these overlapping waves is called a bow wave.
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Sonic Boom:When an airplane flies faster than sound, a continuous loud rumbling is created (sonic
boom).
Other examples of sonic booms include a cracking whip, and a supersonic bullet.
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A 20mm shell flying at 500 m/s. Notice the shock waves produced.
The exposure was only 20 nanoseconds!
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SOUND! Pitch:
Pitch is equivalent to frequency.
high pitch = high frequency
low pitch = low frequency
1000 Hz 1500 Hz 2000 Hz
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Human Hearing:Most humans hear approximately 20Hz through 20,000 Hz. Although this may diminish with age.
The Animals:
As you may know, many animals have higher or lower ranges to their hearing compared
to humans.
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Beyond the Limits:
Sounds of a higher frequency than we can hear are called ultrasonic. ( ultraviolet is higher
frequency than violet light)
Sounds of a lower frequency than we can hear are called infrasonic. (infrared is lower
frequency than red light)
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Nature of Sound in Air:
Sound is simply a longitudinal wave carrying energy that our ears detect.
It is also a mechanical wave – it must have a material /medium to move through!
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Tuning Forks:
A tuning fork provides a continuous push or vibration that creates a particular frequency.
Although you may not be able to see it, the tines actually do move back and forth.
As you may have noticed, the length of the tuning fork helps determine the frequency of its
vibration.
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Other Media:Sound does not only travel in air. In fact, sound
travels faster through most solids or liquids compared to air. The collisions travel better in
more dense media.
H2O molecules more tightly packed
Gas molecules less tightly packed
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Speed of Sound:
Although we usually use 343 m/s for the speed of sound, this number varies because of a
variety of factors.
Wind, temperature, and humidity are all factors that influence the speed of sound.
343 m/s is for 20oC and 1 atmosphere.
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Reflection of Sound:
When sound reflects off a surface, we call it an echo.
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Animal Reflections:Bats and dolphins emit sound that is reflected off objects. The reflected sound is received by
them and allows them to “see” their environment.
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Reverberations:When many sounds echo many times off
multiple surfaces, that is called a reverberation.
You might get this in a large hall that
has no sound dampening.
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Refraction of Sound:
Since sound travels at different speeds in various temperatures of air, sound can bend if there is a layer of different temperature air.
Sometimes at night you can hear very distant objects...
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Natural Frequency:Any elastic material object will
vibrate at one particular natural frequency.
When you lightly tap a crystal glass, you hear its natural frequency.
When an object is forced to vibrate, and this vibration matches its natural frequency, you have
resonance.It is constructive interference since the repeating waves of
the natural frequency add up to produce a larger wave.
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Swing Analogy:Imagine starting to push someone on a swing. You need to push them at just the right time, so that their amplitude continually increases.
A certain bridge was unlucky enough to have its natural frequency match the wind frequency at that location. These wind waves constructively interfered and produced a large enough wave to completely destroy the bridge!
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Beats:A strange effect happens when you add waves
that are just slightly different frequencies.
You get beats, a periodic variation in amplitude.
The two top waves add together,
superimpose, to produce the bottom pattern...