# Physics 20 Questions

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question
How does sound relate to energy?
Sound waves transport energy from one place to another. Sounds are waves produced by vibrations of matter. The original vibration stimulates the vibration of something larger or more massive. This vibrating material then sends a disturbance through the surrounding medium, ex. air, as longitudinal waves.
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What is the relationship between frequency and pitch?
*Pitch* is the human perception of sound frequency. Our subjective impression about the frequency of sound is pitch. *Frequency* corresponds to pitch. A high-pitched sound has high frequency; a low pitch sound has a low frequency.
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What is the average range of a young person's hearing?
20 to 20,000 hertz. As humans age, this hearing range shrinks, especially at the high-frequency end.
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Distinguish between infrasonic and ultrasonic sound waves.
*Infrasonic* is too low in frequency (below 20 Hz) for humans to hear, whereas *ultrasonic* is too high (above 20,000 Hz).
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What is the wave frequency of sound produced by a 220-Hz tuning fork.
220 Hz.
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Relative to solids and liquids, how does air rank as a conductor of sound?
Relative to solids and liquids, air is a poor conductor of sound. Any elastic substance, solid liquid gas plasma, can transmit sound. Inelastic substances (putty) cannot transmit sound. The speed of sound is generally greater in solids than in liquids, and greater in liquids than in gases.
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Why won't sound travel in a vacuum?
There is no material in a vacuum to vibrate and carry the sound. Vacuums do not transmit sound since there is nothing to compress or expand.
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Distinguish between a compression and a rarefaction.
In a compression, the molecules are closer together than average; in a rarefaction, they are further apart.
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Do compressions and rarefactions travel in the same direction, or in opposite directions, in a wave?
They travel in the same direction at the same speed.
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What factors does the speed of sound depend upon? What are some factors that it does not depend upon?
It depends on wind conditions, temperature and humidity, not on loudness and frequency. All sounds in the same medium travel at the same speed.
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What is the speed of sound in dry air at 20ยฐC?
340 m/s
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Does sound travel faster in warm air or in cold air? In humid air or dry air?
Faster in warm air, faster in humid air. Water vapor in the air increases the speed slightly. Faster moving molecules in warm air bump into each other more often and transmit a pulse in less time.
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Do the compressions and rarefractions in a sound wave travel in the same direction or in opposite directions from one another?
They travel in the same direction, same speed, as shown in Fig. 20.1
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What is the approximate distance of a thunderstorm when you note a 3 second delay between the flash of lightning and the sound of thunder?
Assuming that the speed of sound in air is about 340 m/s, in 3 seconds it will travel 340 m/s X 3 seconds = 1020 m. There is no appreciable delay for the flash, so the storm is slightly more than 1 km away.
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Which is normally greater, the energy in ordinary sound or the energy in ordinary light? How does the speed of sound compare to the speed of light?
The energy in ordinary light is greater than the energy in ordinary sound. The speed of sound is a million times less than the speed of light.
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What ultimately happens to the energy of sound in the air?
Sound energy dissipates to thermal energy while sound travels in air. For waves of higher frequency, the sound energy is transformed into internal energy more rapidly than low frequency waves. Thus low frequency sounds like a fog horn travel farther through air.
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What is an echo?
An echo is the reflection of sound. The fraction of energy carried by the reflected sound wave can be large with a rigid smooth surface, and small with a soft irregular surface.
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What is a reverberation?
When sound undergoes multiple reflections yet still persists after the source has ceased emitting.
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What is the cause of refraction?
Different speeds of sound in a medium.
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Does sound tend to bend upward or downward when its speed is less near the ground?
Downward. When the speed of sound near the ground is reduced, (such as on a cold day when the layer of air nearest ground is colder than the air above) the higher speed of the wavefronts above cause a bending of sound toward the ground. This is sound clearly heard over considerably longer distances.
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Why does sound sometimes refract under water?
Changing temperature changes the speed of sound in water. This is problematic for surface vessels that bounce high-frequency waves off the ocean bottom to chart. But its a feature that subs avoiding detection take advantage of!
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What is ultrasound?
Sound above the frequency of human hearing. Ultrasound is composed of frequencies higher than human hearing range.
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An oceanic depth-sounding vessel surveys the ocean bottom with ultrasonic sound that travels at an average of 1530 m/s in seawater. How deep is the water if the time delay of the echo from the ocean floor is 2 s?
1530 meters: 1 second up, and 1 second down.
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Why will a struck tuning fork sound louder when it is held against a table?
The table is forced to vibrate, and its larger surface area vibrates more air than the tuning fork. This is forced vibration.
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Which two factors determine the natural frequency of an object?
Elasticity and shape of the object. It will vibrate at its own characteristic frequency. Ex. a wrench and baseball bat make different sounds when hitting the floor. Each has its own natural frequency.
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How do forced vibrations relate to resonance?
When the frequency of forced vibrations match an object's natural frequency, resonant amplification occurs. It does need a force to pull it back into its starting position, and enough energy to keep it vibrating. Ex. a child's swing
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When you listen to a radio, why do you hear only one station at a time rather than all stations at once?
The natural frequency of the electronics is tuned to resonate with the frequency of one station.
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How did wind-generated resonance affect the Tacoma Narrows Bridge in the state of Washington in 1940?
The resonant amplification of bridge motion destroyed the bridge. Wind produced an irregular force in resonance with the natural frequency of the bridge, steadily increasing the amplitude of vibration until the bridge collapsed.
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When is it possible for one wave to cancel another?
When both waves have the same amplitude and when the crest of one wave overlaps the trough of another. Destructive wave interference occurs when two identical waves, out of phase, are superimposed and destroy each other.
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What kind of waves can exhibit interference?
All waves (refer to Chapter 19!)
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What is the result of a mirror image of a sound signal combining with the sound itself?
The sound is cancelled when added to its mirror image. Ex. The mirror image wave pattern of the sound signal is combined with the actual sound, and cancels out the noise.
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What physical phenomenon underlies the production of beats?
Beats are produced by interference of two waves of slightly different frequencies, heard as a throbbing effect in sound waves.
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What beat frequency will occur when a 370-Hz and a 374-Hz sound source are sounded together?
*4 Hz*
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What is the beat frequency when a 262-Hz tuning fork and a 266-Hz tuning fork are sounded together?
the ear will hear 264 Hz, which will be at *4 Hz* (266 Hz - 262 Hz)
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What is the beat frequency when a 262-Hz tuning fork and a 272 Hz tuning fork are sounded together?
267 Hz will be heard, and some people will hear it throb *10* times each second (272 Hz - 262 Hz). Beat frequencies higher than 10 Hz are normally too rapid to be heard.
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How does a radio wave differ from a sound wave?
A radio wave is an electromagnetic wave that travels at the speed of *light*, whereas a sound wave travels at the speed of *sound* through air or another elastic medium.
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When a sound wave moves past a point in air, are there changes in the density of air at this point?
yes, the sound wave is a pressure wave (an increase in pressure) followed by a low pressure (reduction in pressure) trough, relative to normal air pressure. After the sound wave passes, the air it passed through returns to its original pressure.
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If the speed of sound were dependent on its frequency, would you enjoy a concert sitting in the second balcony?
not as many waves would reach you, and you couldn't hear the concert!
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If a single disturbance at an unknown distance emits both transverse and longitudinal waves that travel with distinctly different speeds in the medium, such as in the ground during an earthquake, how can the distance to the disturbance be determined?
Earthquakes emit both pressure and shear waves (known as P and S waves); P waves travel faster through the Earth than S waves. When P and S waves reach a seismic station, they leave characteristic traces on the seismograph, so it is easy for an experienced user to determine the time of arrival of the first P wave and of the first S wave. Calibrate how far away the earthquake was given the P-S arrival time difference.
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What two physics mistakes occur in a science fiction movie that shows a distant explosion in outer space, where you see and hear the explosion at the same time?
Space is a vacuum so the explosion has no sound and light travels faster than sound so we would see the explosion long before we heard it.
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Apartment dwellers will testify that bass notes are more distinctly heard from music played in nearby apartments. Why do you suppose lower-frequency sounds travel through walls, floors, and ceilings more easily?
Sounds of lower frequencies travel farther through air than sounds of higher frequencies. Also, solids conduct sound better than air. The natural frequency of large walls, floors, and ceilings is lower than the natural frequency of smaller surfaces; bass notes more easily set them into forced vibrations and resonance.
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Two sound waves of the same frequency can interfere, but, in order to produce beats, the two sound waves must be of different frequencies. Why?