Physics chapter 20

An object resonates when the frequency of a vibrating force either matches its natural frequency or is a submultiple of its natural frequency. Why won't it resonate to multiples of its natural frequency?

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Why is the sound of a harp soft in comparison with the sound of a piano?

A harp produces relatively softer sounds than a piano because its sounding board is smaller and lighter

At the instant that a high-pressure region is created just outside the prongs of a vibrating tuning fork, what is being created inside between the prongs?

At the instant that a high-pressure region is created just outside the prongs of a tuning fork, a low pressure region is created between the prongs.

Rank the beat frequencies from highest to lowest for the following pairs of sounds. A) 132 Hz, 136 Hz B) 264 Hz, 258 Hz C) 528 Hz, 531 Hz D) 1056 Hz, 1058 Hz

B, A, C, D

Why does sound travel slower in cold air than in warm air?

Because in warm air the molecules are hitting each other faster while in cold air the molecules are hitting each other at slower rates.

In Olympic competition, a microphone detects the sound of the starter's gun and sends it electrically to speakers at every runner's starting block. Why?

Because the speed of sound is much slower than the speed of electricity, this reduces the split-second delay between firing the gun and the sound reaching the runner furthest away from the gun.

Why does a dance floor heave only when certain kinds of dance steps are being performed?

Difference dance steps have different frequencies (and periods), but the dance floor has one frequency at which it resonates - because of the size of the dance floor, the material it is made out of, etc. When everybody dance at the same frequency as the resonant frequency of the dance floor, and they move together (everybody up and down at the same time), then the energy from their dancing is most efficiently transferred into the dance floor. At that point, the floor can move quite a bit.

If the Moon blew up, why wouldn't we hear it?

For sound to travel, it needs air as a medium. Since there is no air in space, sound can't travel. Thats why astronauts use radios to communicate.

A pair of loudspeakers on two sides of a stage are emitting identical pure tones. When you stand in the center aisle, equally distant from the two speakers, you hear the sound loud and clear. Why does the intensity of the sound diminish considerably when you step to one side?

You are in a region of destructive interference.

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 te distance to the disturbance be determined?

If a single disturbance at some unknown distance sends longitudinal waves at one known speed, and transverse waves at a lesser-known speed, and you measure the difference in time of the waves as they arrive, you can calculate the distance. The wider the gap in time, the greater the distance—which could be in any direction. If you use this distance as the radius of a circle on a map, you know the disturbance occurred somewhere on that circle. If you telephone two friends who have made similar measurements of the same event from different locations, you can transfer their circles to your map, and the point where the three circles intersect is the location of the disturbance.

Ultrasonic waves have many applications in technology and medicine. One advantage is that large intensities can be used without danger to the ear. Cite another advantage of their short wavelength.

If an object is smaller than a wave's wavelength, then the wave will simply defract around the object, instead of reflecting off of it (reflection allows you to see objects). This is why objects smaller than the wavelength of visible light do not have color and cannot be seen with light microscopes. - So, the small wavelength of ultrasonic waves means that they can reflect off of very small objects, allowing us to see them.

Two sound waves of the same frequency can interfere, but, in order to roduce beats, the two sound waves must have different frequencies. Why?

If sound waves have the same frequency, they can be totally in phase, totally out of phase, or anything in between, but their waves always either reinforce or cancel each other, so that the "summed sound" is either louder or softer than the individual sounds. But if the two sound waves are of different frequencies, they cancel each other except when their crests coincide; this amplification produces the "beat" phenomenon.

If the speed of sound were dependent on its frequency, would you enjoy a concert sitting in the second balcony? Explain.

If the speed of sound was different for different frequencies, say, faster for higher frequencies, then the farther a listener is from the music source, the more jumbled the sound would be. In that case, higher-frequency notes would reach the ear of the listener first. The fact that this jumbling doesn't occur is evidence that sound of all frequencies travel at the same speed.

Are beats the result of interference, of the Doppler effect, or of both?

Interference

Why will marchers at the end of a long parade following a band be out of step with marchers near the front?

Marchers at the end of a long parade will be out of step with marchers nearer the band because time is required for the sound of the band to reach the marchers at the end of a parade. They will step to the delayed beat they hear.

If you toss a stone in still water, concentric circles are formed. What form will waves have if a stone is tossed into smoothly flowing water?

Not counting effects from the air, it will still be concentric circles (principle of relativity).

If the distance from a bugle is tripled, by what factor does the sound intensity decrease? Assume that no reflections affect the sound.

Sound intensity is related to distance from the source by the inverse square law (like light or gravity) so a factor of 1/3^2=1/9

A cat can hear sound frequencies up to 70,000 Hz. Bats send and recieve ultra-high-frequencies squeaks up to 120,000 Hz. Which hears sound of shorter wavelengths:cats or bats?

The bat because the larger the frequency, the shorter the wavelength.

If a bell is ringing inside a bell jar, we can no longer hear it when the air is evacuated, but we can still see it. What differences in the properties of sound and light does this indicate?

The fact that we can see a ringing bell but can't hear it indicates that light is a distinctly different phenomenon than sound. When we see the vibrations of the "ringing" bell in a vacuum, we know that light can pass through a vacuum. The fact that we can't hear the bell indicates that sound does not pass through a vacuum. Sound needs a material medium for its transmission; light does not.

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 than high frequency sounds?

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.

A piano tuner using a 264 Hz tuning fork hear 4 beats per second. What are two possible frequencies of vibration of the piano wire?

The piano tuner should loosen the piano string. When 3 beats per second is first heard, the tuner knows he was 3 hertz off the correct frequency. But this could be either 3 hertz above or 3 hertz below. When he tightened the string and increased its frequency, a lower beat frequency would have told him he was on the right track. But the greater beat frequency told him he should have been loosening the string. When there is no beat frequency, the frequencies match.

As you pour water into a glass, you repeatedly tap the glass with a spoon. As the tapped glass is being filled, does the pitch of the sound increase or decrease?

The pitch of the sound decreases. Theoretical justification: Because the column of water increases, therefore the sound waves have a greater length of water to resonate through. This increases their wavelength, which lowers their frequency, which produces a sound that has a lower pitch.

What happens to the wavelength of sound as the frequency increases?

The wavelengths decrease

Suppose a sound wave and an electromagnetic wave have the same frequency. Which has the longer wavelength?

They have the same length of wavelengths

Why can the tremor of the ground from a distant explosion be felt before the sound of the explosion can be heard?

his is similar to lightning and thunder. You see the lightning first, then you here the thunder, because the speed of light is faster than the speed of sound. Well, tremors are slower than light, but faster than sound.