Ch. 16

Question 1

Considering the nature of a water wave, which of the following statements correctly describes how a fishing float moves on the surface of a lake when a wave passes beneath it?

Answer 1

It moves in a vertical plane, exhibiting both motions described in A and B simultaneously

A) it bobs up and down vertically
B) it moves back and forth horizontally

Question 2

Pulling slinky back and forth

Suppose that the longitudinal wave moves to the right at a speed of 1m/s. Does one coil of the slinky move a distance of 1mm to the right in a time of 1ms?

Answer 2

No. The coil moves back and forth in simple harmonic motion.

Question 3

A sound wave (a periodic longitudinal wave) from a loudspeaker travels from air into water. The frequency of the wave does not change, because the loudspeaker producing the sound determines the frequency. The speed of sound in air is 343m/s, whereas the speed in fresh water is 1482m/s. When the sound wave enters the water, does its wavelength increase, decrease, or remain the same?

Answer 3

The wavelength increases

Question 4

One end of each of two identical strings is attached to a wall. Each string is being pulled equally right by someone at the other end. A transverse pulse is sent traveling along string A. A bit later an identical pulse is sent traveling along string B. What, if anything, can be done to make the pulse on string B catch up with and pass the pulse on string A?

Answer 4

The person pulling on string B should pull harder to increase the tension in the string.

Question 5

The concept of a massless rope is discussed. Considering the equation, would it take any time for a transverse wave to travel the length of a truly massless rope?

v= sqrt( F/ m/L )

Answer 5

The speed would be so infinitely large if m were zero, so it would take no time at all.

Question 6

A wire is strung tightly between two immovable posts. Decide whether the speed of a transverse wave on this wire would increase, decrease, or remain the same when the temperature increases. Ignore any change in the mass per unit length of the wire.

Answer 6

Decrease

Question 7

A wave moves on a string with a constant velocity. Does this mean that the particles of the string always have zero acceleration?

Answer 7

No, because the particles exhibit simple harmonic motion, in which the acceleration is not always zero.

Question 8

A rope of mass m is hanging down from the ceiling. Nothing is attached to the loose end of the rope. As a transverse wave travels upward on the rope, does the speed of the wave increase, decrease, or remain the same?

Answer 8

Increase

Question 9

In a traveling sound wave, are there any particles that are always at rest as the wave passes by?

Answer 9

No, because each particle executes simple harmonic motion as the wave passes by.

Question 10

Do you expect an echo to return to you more quickly on a hot day or a cold day, other things being equal?

Answer 10

Hot day

Question 11

Carbon monoxide (CO), hydrogen (H), and nitrogen (N2) may be treated as ideal gases. Each has the same temperature and nearly the same value for the ratio of the specific heat capacities at constant pressure and constant volume. In which two of the three gases is the speed of sound approximately the same?

Answer 11

CO and N2

Question 12

Jell-O starts out as a liquid and then sets to a gel. As the Jell-O sets and becomes more solid, does the speed of sound in this material increase, decrease, or remain the same?

Answer 12

Increase

Question 13

Some animals rely on an acute sense of hearing for survival, and the visible parts of the ears in such animals are often relatively large. How does this anatomical feature help to increase the sensitivity of the animals hearing for low-intensity sounds?

Answer 13

Large outer ears intercept and direct more sound power into the auditory system than smaller ones do.

Question 14

A source is emitting sound uniformly in all directions. There are no reflections anywhere. A flat surface faces the source. Is the sound intensity the same at all points on the surface?

Answer 14

No, because not all points on the surface are at the same direction from the source.

Question 15

If two people talk simultaneously and each creates an intensity level of 65dB at a certain point, does the total intensity level at this point equal 130dB?

Answer 15

No, because it is the intensities I1 and I2 that add to give a total intensity Itotal. The intensity levels B1 and B1 do not add to give a total intensity level Btotal.

Question 16

At a swimming pool, a music fan up on a diving platform is listening to a radio. As the radio is playing a tone that has a constant frequency fs, is it accidentally knocked off the platform. Describe the Doppler effect heard by a) the person on the platform and b) a person down below in the water. In each case, state whether the observed frequency fo is greater or smaller than fs and describe how fo changes as the radio falls.

Answer 16

a) fo is smaller than fs, and fo decreases during the fall

b) fo is greater than fs, and fo increases during the fall

Question 17

When a car is at rest, its horn emits a frequency of 600Hz. A person standing in the middle of the street with this car behind him hears the horn with a frequency of 580Hz. Does he need to jump out of the way?

Answer 17

No, because the observed frequency is less than the source frequency, so the car is moving away from him

Question 18

A source of sound produces the same frequency under water as it does in air. The source has the same velocity in air as it does under water. Consider the ratio fo/fs of the observed frequency fo to the source frequency fs. Is the ratio greater in air or under water when the source a) approaches and b) moves away from the observer?

Answer 18

a) greater in air

b) greater under water

Question 19

Two cars, one behind the other, are traveling in the same direction at the same speed. Does either driver hear the others horn at a frequency that is different from the frequency heard when both cars are at rest?

Answer 19

No, because there is no relative motion of the cars

Question 20

When a truck is stationary, it’s horn produces a frequency of 500Hz. You are driving your car, and this truck is following behind. You hear it’s horn at a frequency of 520Hz. a) refer to equation 16.15 and decide which algebraic sign should be used in the numerator and which in the denomination. b) which driver, if either, is driving faster?

Answer 20

a) minus sign in both places

b) the truck driver

Question 21

Domino toppling is an event in which a large number of dominoes are lined up close together and then allowed to topple, one after the other. The disturbance that propagates along the line of dominoes is ____.

Answer 21

Partly transverse and partly longitudinal

As one domino toppled against the next one in line, it is moving partly perpendicular and partly parallel to the direction along which the disturbance propagates.

Question 22

A transverse wave on a string has an amplitude A. A tiny spot on the string is colored red. As one cycle of the wave passes by, what is the total distance traveled by the red spot?

Answer 22

4A

Question 23

As a wave moves through a medium at a speed v, the particles of the medium move in simple harmonic motion about their undisturbed positions. The maximum speed of the simple harmonic motion is vmax. When the amplitude of the wave doubles, ___.

Answer 23

v remains unchanged, but vmax doubles

Question 24

A rope is attached to a hook in the ceiling and is hanging straight down. The rope has a mass m, and nothing is attached to the free end of the rope. As a transverse wave travels down the rope from the top, ___.

Answer 24

The speed of the wave decreases

Question 25

As the amplitude of a sound wave in air decreases to zero, ___.

Answer 25

Both the condensations and the rarefactions of the wave disappear.

Question 26

An echo is sound that returns to you after being reflected from a distant surface. Assuming that the distances involved are the same, an echo under water and an echo in air return to you ___.

Answer 26

At different times, the echo under water returning more quickly.

Question 27

A horn on a boat sounds a warning, and the sound penetrates the water. How does the frequency of the sound in the air compare to its frequency in the water? How does the wavelength in the air compare to the wavelength in the water?

Answer 27

The frequency in the air is the same as the frequency in the water.

The wavelength in the air is smaller than the wavelength in the water.

Question 28

A red car and a blue car can move along the same straight one-lane road. Both cars can move only at one speed when they move. The driver of the red car sounds his horn. In which one of the following situations does the driver of the blue car hear the highest horn frequency?

Answer 28

Both cars are moving at the same speed, and they are moving toward each other.

Question 29

What happens to the Doppler effect in air as the temperature increases?

Answer 29

It is less at higher temperatures than at lower temperatures