Conceptual Physics Fundamentals

Paul G. Hewitt

Chapter 12

Waves and Sound - all with Video Answers

Educators

Chapter Questions

Problem 1

What is the source of wave motion?

Eduard Sanchez

Eduard Sanchez

Numerade Educator

Problem 2

If we double the frequency of a vibrating object, what happens to its period?

Darshan Maheshwari

Darshan Maheshwari

Numerade Educator

Problem 3

You dip your finger repeatedly into a puddle of water and make waves. What happens to the wavelength if you dip your finger more frequently?

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Keyan Sheppard

Numerade Educator

Problem 4

How does the frequency of vibration of a small object floating in water compare to the number of waves passing it each second?

Averell Hause

Carnegie Mellon University

Problem 5

What kind of motion should you impart to the nozzle of a garden hose so that the resulting stream of water approximates a sine curve?

Shital Rijal

Numerade Educator

Problem 6

What kind of motion should you impart to a stretched coiled spring (or to a Slinky) to produce a transverse wave? A longitudinal wave?

Alex Garger

Numerade Educator

Problem 7

If a gas tap is turned on for a few seconds, someone a couple of meters away will hear the gas escaping long before he or she smells it. What does this indicate about the speed of sound and the motion of molecules in the sound-carrying medium?

Manish Jain

Numerade Educator

Problem 8

A cat can hear sound frequencies up to $70,000 \mathrm{~Hz}$. Bats send and receive ultrahigh-frequency squeaks up to $120,000 \mathrm{~Hz}$. Which hears sound of shorter wavelengths, cats or bats?

Prabhu Ramji

Numerade Educator

Problem 9

What does it mean to say that a radio station is "at 101.1 on your FM dial"?

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 10

Sound from Source A has twice the frequency of sound from Source B. Compare the wavelengths of sound from the two sources.

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 11

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

Prabhu Ramji

Numerade Educator

Problem 12

At the stands of a racetrack, you notice smoke from the starter's gun before you hear it fire. Explain.

Prabhu Ramji

Numerade Educator

Problem 13

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

Abhishek Jana

Numerade Educator

Problem 14

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?

Prabhu Ramji

Numerade Educator

Problem 15

Why is it so quiet after a snowfall?

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 16

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

Abhishek Jana

Numerade Educator

Problem 17

Why is the Moon described as a "silent planet"?

Prabhu Ramji

Numerade Educator

Problem 18

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? (What should you do to answer this question?)

Prabhu Ramji

Numerade Educator

Problem 19

If the speed of sound depended on its frequency, would you enjoy a concert sitting in the second balcony?

Abhishek Jana

Numerade Educator

Problem 20

If the frequency of sound is doubled, what change will occur in its speed? What change will occur in its wavelength? Defend your answer.

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 21

Why does sound travel faster in warm air?

Prabhu Ramji

Numerade Educator

Problem 22

Why does sound travel faster in moist air? (Hint: At the same temperature, water-vapor molecules have the same average kinetic energy as the heavier nitrogen and oxygen molecules in the air. How, then, do the average speeds of $\mathrm{H}_{2} \mathrm{O}$ molecules compare with those of $\mathrm{N}_{2}$ and $\mathrm{O}_{2}$ molecules?)

Prabhu Ramji

Numerade Educator

Problem 23

Why is an echo weaker than the original sound?

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 24

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?

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 25

A rule of thumb for estimating the distance in kilometers between an observer and a lightning stroke is to divide the number of seconds in the interval between the flash and the sound by $3 .$ Is this rule correct?

Prabhu Ramji

Numerade Educator

Problem 26

If a single disturbance some unknown distance away sends out both transverse and longitudinal waves that travel with distinctly different speeds in the medium, such as in the ground during an earthquake, how could the distance to the disturbance be determined?

Abhishek Jana

Numerade Educator

Problem 27

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

Abhishek Jana

Numerade Educator

Problem 28

What is the danger posed by people in the balcony of an auditorium stamping their feet in a steady rhythm?

Katrina Olenwine

Katrina Olenwine

Numerade Educator

Problem 29

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

Abhishek Jana

Numerade Educator

Problem 30

If the handle of a tuning fork is held solidly against a tabletop, the sound from the tuning fork becomes louder. Why? How will this affect the length of time the fork keeps vibrating? Explain.

Prabhu Ramji

Numerade Educator

Problem 31

The sitar, an Indian musical instrument, has a set of strings that vibrate and produce music, even though the player never plucks them. These "sympathetic strings" are identical to the plucked strings and are mounted below them. What is your explanation?

Prabhu Ramji

Numerade Educator

Problem 32

A special device can transmit sound that is out of phase with the sound of a noisy jackhammer to the jackhammer operator by means of earphones. Over the noise of the jackhammer, the operator can easily hear your voice while you are unable to hear his. Explain.

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 33

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

Abhishek Jana

Numerade Educator

Problem 34

Walking beside you, your friend takes 50 strides per minute while you take 48 strides per minute. If you start in step, when will you be in step again?

Prabhu Ramji

Numerade Educator

Problem 35

Suppose a piano tuner hears three beats per second when listening to the combined sound from his tuning fork and the piano note being tuned. After slightly tightening the string, he hears five beats per second. Should the string be loosened or tightened?

Eric Mockensturm

Eric Mockensturm

Numerade Educator

Problem 36

A railroad locomotive is at rest with its whistle shrieking, and then it starts moving toward you.
a. Does the frequency that you hear increase, decrease, or stay the same?
b. How about the wavelength reaching your ear?
c. How about the speed of sound in the air between you and the locomotive?

Averell Hause

Carnegie Mellon University

Problem 37

When you blow your horn while driving toward a stationary listener, an increase in frequency of the horn is heard by the listener. Would the listener hear an increase in the frequency of the horn if he were also in a car traveling at the same speed in the same direction as you are? Explain.

Averell Hause

Carnegie Mellon University

Problem 38

How does the Doppler effect aid police in detecting speeding motorists?

Averell Hause

Carnegie Mellon University

Problem 39

Astronomers find that light emitted by a particular element at one edge of the Sun has a slightly higher frequency than light from that element at the opposite edge. What do these measurements tell us about the Sun's motion?

Alex Garger

Numerade Educator

Problem 40

Would it be correct to say that the Doppler effect is the apparent change in the speed of a wave due to motion of the source? (Why is this question a test of reading comprehension as well as a test of physics knowledge?)

Shital Rijal

Numerade Educator

Problem 41

Does the conical angle of a shock wave open wider, narrow down, or remain constant as a supersonic aircraft increases its speed?

Eduard Sanchez

Numerade Educator

Problem 42

If the sound of an airplane does not originate in the part of the sky where the plane is seen, does this imply that the airplane is traveling faster than the speed of sound? Explain.

Eduard Sanchez

Numerade Educator

Problem 43

Does a sonic boom occur at the moment when an aircraft exceeds the speed of sound? Explain.

Averell Hause

Carnegie Mellon University

Problem 44

Why is it that a subsonic aircraft, no matter how loud it may be, cannot produce a sonic boom?

Shital Rijal

Numerade Educator

Problem 45

What physics principle is used by Manuel when he pumps in rhythm with the natural frequency of the swing?

Narayan Hari

Numerade Educator