• Home
  • Textbooks
  • College Physics: A Strategic Approach
  • Traveling Waves and Sound

College Physics: A Strategic Approach

Randall D. Knight, Brian Jones, Stuart Field

Chapter 15

Traveling Waves and Sound - all with Video Answers

Educators

Chapter Questions

02:07

Problem 1

The wave speed on a string under tension is 200 m/s. What is the speed if the tension is doubled?

Vishal Gupta
Vishal Gupta
Numerade Educator
01:23

Problem 2

The wave speed on a string is 150 m/s when the tension is 75.0 N. What tension will give a speed of 180 m/s?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:34

Problem 3

The back wall of an auditorium is 26.0 m from the stage. If you are seated in the middle row, how much time elapses between a sound from the stage reaching your ear directly and the same sound reaching your ear after reflecting from the back wall?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:54

Problem 4

Bats sense objects in the dark by echolocation, in which they emit very short pulses of sound and then listen for their echoes off the objects. A bat is flying directly toward a wall 50 m away when it emits a pulse. 0.28 s later it receives the pulse. What is the bat’s speed?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:31

Problem 5

A scientist measures the speed of sound in a monatomic gasto be 449 m/s at 20°C. What element does this gas consist of?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:24

Problem 6

A medical ultrasound imaging system sends out a steady stream of very short pulses. To simplify analysis, the reflection of one pulse should be received before the next is transmitted. If the system is being used to create an image of tissue 12 cm below the skin, what is the minimum time between pulses?
How many pulses per second does this correspond to?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:40

Problem 7

An earthquake 45 km from a city produces P and S waves that travel outward at 5000 and 3000 m/s, respectively. Once city residents feel the shaking of the P wave, how much time do they have before the S wave arrives?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:40

Problem 8

A stationary boat in the ocean is experiencing waves from a storm. The waves move at 56 km/h and have a wavelength of 160 m, both typical values. The boat is at the crest of a wave. How much time elapses until the boat is first at the trough of a wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:49

Problem 9

Figure $\mathrm{P} 15.9$ is a snapshot graph of a wave at $t=0 \mathrm{s}$. Draw the history graph for this wave at $x=6 \mathrm{m},$ for $t=0 \mathrm{s}$ to $6 \mathrm{s}$.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:19

Problem 10

Figure Q15.10 shows a history graph and a snapshot graph for
a wave pulse on a string. They describe the same wave from
two perspectives.
a. In which direction is the wave traveling? Explain.
b. What is the speed of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:16

Problem 11

Rank in order, from largest to smallest, the wavelengths $\lambda_{1}$ to $\lambda_{3}$ for sound waves having frequencies $f_{1}=100 \mathrm{Hz}, f_{2}=1000 \mathrm{Hz}$ and $f_{3}=10,000 \mathrm{Hz}$. Explain.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:26

Problem 12

Figure P15.12 is a history graph at x = 0 m of a wave moving to the right at 1 m/s. Draw a snapshot graph of this wave at t = 1 s.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
04:18

Problem 13

II Figure $P 15.13$ is a history graph at $x=2 \mathrm{m}$ of a wave moving to the left at $1 \mathrm{m} / \mathrm{s}$. Draw the snapshot graph of this wave at $t=0 \mathrm{s}$

Vedad Babic
Vedad Babic
Numerade Educator
02:51

Problem 14

| Figure P15.14 shows history graphs of two different points on a string as a wave pulse moves along the string. The blue curve is the history graph for the point at x = 1.0 cm, and the green curve is for the point at x = -3.0 cm. What is the velocity (including the correct sign for its direction) of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:41

Problem 15

A sinusoidal wave has period 0.20 s and wavelength 2.0 m. What is the wave speed?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:45

Problem 16

A sinusoidal wave travels with speed 200 m/s. Its wavelength is 4.0 m. What is its frequency?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:54

Problem 17

The motion detector used in a physics lab sends and receives 40 kHz ultrasonic pulses. A pulse goes out, reflects off the object being measured, and returns to the detector. The lab temperature is 20°C.
a. What is the wavelength of the waves emitted by the motion
detector?
b. How long does it take for a pulse that reflects off an object
2.5 m away to make a round trip?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:46

Problem 18

The displacement of a wave traveling in the positive $x$ -direction is $y(x, t)=(3.5 \mathrm{cm}) \times \cos (2.7 x-92 t),$ where $x$ is in $\mathrm{m}$ and $t$ is in $\mathrm{s}$. What are the
(a) frequency,
(b) wavelength, and
(c) speed of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
04:08

Problem 19

III A traveling wave has displacement given by $y(x, t)=$ $(2.0 \mathrm{cm}) \times \cos (2 \pi x-4 \pi t),$ where $x$ is measured in $\mathrm{cm}$ and $t$ in $\mathrm{s}$
a. Draw a snapshot graph of this wave at $t=0$ s.
b. On the same set of axes, use a dotted line to show the snaphbot graph of the wave at $t=1 / 8 \mathrm{s}$
What is the speed of the wave?

Supratim Pal
Supratim Pal
Numerade Educator
01:01

Problem 20

Figure $P 15.20$ is a snapshot graph of a wave at $t=0$ s. What are the amplitude, wavelength, and frequency of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:50

Problem 21

The probe used in a medical ultrasound examination emits sound waves in air that have a wavelength of 0.12 mm. What is the wavelength of the sound waves in the patient?
A. 0.027 mm B. 0.12 mm C. 0.26 mm D. 0.54 mm

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:21

Problem 21

Figure P15.21 is a history graph at x = 0 m of a wave moving to the right at 2 m/s. What are the amplitude, frequency, and
wavelength of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:11

Problem 22

Ultrasound can be used to deliver energy to tissues for therapy. It can penetrate tissue to a depth approximately 200 times its wavelength. What is the approximate depth of penetration of ultrasound at a frequency of 5.0 MHz?
A. 0.29 mm B. 1.4 cm C. 6.2 cm D. 17 cm

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:51

Problem 22

Figure P15.22 shows snapshot (left) and history (right) graphs for a wave traveling on a string. The snapshot graph shows the wave at t = 0 s; the history graph shows the displacement of the point on the string at x = 1.5 cm, indicated by the dot in the snapshot graph. What is the speed of the wave, and in
which direction is it traveling?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:53

Problem 23

A sinusoidal wave traveling on a string has a period of 0.20 s, a wavelength of 32 cm, and an amplitude of 3 cm. The speed of this wave is
A. 0.60 cm/s. B. 6.4 cm/s. C. 15 cm/s. D. 160 cm/s.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:52

Problem 23

A sinusoidal wave moving to the left has a wavelength of $5.0 \mathrm{cm}$ and a frequency of $50 \mathrm{Hz}$ At $t=0 \mathrm{s},$ the wave has a crest at $x=0 \mathrm{cm} .$ What is the earliest time after $t=0 \mathrm{s}$ at which there is a crest at the position $x=3.0 \mathrm{cm} ?$

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:36

Problem 24

Two strings of different linear density are joined together and pulled taut. A sinusoidal wave on these strings is traveling to the right, as shown in Figure Q15.24. When the wave goes across the boundary from string 1 to string 2, the frequency is unchanged. What happens to the velocity?
A. The velocity increases.
B. The velocity stays the same.
C. The velocity decreases.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:51

Problem 24

People with very good pitch discrimination can very quickly determine what note they are listening to. The note on the musical scale called C6 (two octaves above middle C) has a frequency of 1050 Hz. Some trained musicians can identify this note after hearing only 12 cycles of the wave. How much time does this correspond to?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:08

Problem 25

You stand at x = 0 m, listening to a sound that is emitted at frequency fs. Figure Q15.25 shows the frequency you hear during a four-second interval. Which of the following describes the motion of the sound source?
A. It moves from left to right and passes you at t = 2 s.
B. It moves from right to left and passes you at t = 2 s.
C. It moves toward you but doesn’t reach you. It then reverses
direction at t = 2 s.
D. It moves away from you until t = 2 s. It then reverses direction and moves toward you but doesn’t reach you.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:46

Problem 25

A dolphin emits ultrasound at 100 kHz and uses the timing of reflections to determine the position of objects in the water. What is the wavelength of this ultrasound?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:37

Problem 26

Elephants can communicate over distances as far as 6 km by using very low-frequency sound waves. What is the wavelength of a 10 Hz sound wave emitted by an elephant?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:48

Problem 27

a. What is the frequency of blue light that has a wavelength
of 450 nm?
b. What is the frequency of red light that has a wavelength
of 650 nm?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:52

Problem 28

Research vessels at sea can create images of their surroundings by sending out sound waves and measuring the time until they detect echoes. This image of a shipwreck on the ocean bottom was made from the surface with 600 kHz ultrasound.
a. What was the wavelength?
b. How deep is the shipwreck if echoes were detected 0.42 s
after the sound waves were emitted?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:37

Problem 29

A bullet shot from a rifle travels at 1000 m/s. What is the elapsed time between when the bullet strikes a target 500 m away, and when the sound of the gunshot reaches the target?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:39

Problem 30

a. An FM radio station broadcasts at a frequency of
101.3 MHz. What is the wavelength?
b. What is the frequency of a sound source that produces
the same wavelength in 20°C air?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:19

Problem 31

Sound is detected when a sound wave causes the eardrum to vibrate (see Figure 14.26 ). Typically, the diameter of the eardrum is about $8.4 \mathrm{mm}$ in humans. When someone speaks to you in a normal tone of voice, the sound intensity at your ear is approximately $1.0 \times 10^{-6} \mathrm{W} / \mathrm{m}^{2} .$ How much energy is delivered to your eardrum each second?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:22

Problem 32

At a rock concert, the sound intensity 1.0 m in front of the bank of loudspeakers is 0.10 W/m2 .A fan is 30 m from the loudspeakers. Her eardrums have a diameter of 8.4 mm. How much sound energy is transferred to each eardrum in 1.0 second?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:08

Problem 33

Ill The intensity of electromagnetic waves from the sun is $1.4 \mathrm{kW} / \mathrm{m}^{2}$ just above the earth's atmosphere. Eighty percent of this reaches the surface al noon on a clear summer day. Suppose you model your back as a $30 \mathrm{cm} \times 50 \mathrm{cm}$ rectangle. How many joules of solar energy fall on your back as you work on your tan for $1.0 \mathrm{h} ?$

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:44

Problem 34

A sun-like star is barely visible to naked-cye observers on earth when it is a distance of 7.0 light years, or $6.6 \times 10^{16} \mathrm{m},$ away. The sun emits a power of $3.8 \times 10^{28} \mathrm{W}$. Using this information, at what distance would a candle that emits a power of $0.20 \mathrm{W}$ just be visible?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:12

Problem 35

A large solar panel on a spacecraft in earth orbit produces 1.0 kW of power when the panel is turned toward the sun. What power would the solar cell produce if the spacecraft were in orbit around Saturn, 9.5 times as far from the sun?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:55

Problem 36

| Solar cells convert the energy of incoming light to electric energy; a good quality cell operates at an efficiency of 15%. Each person in the United States uses energy (for lighting, heating, transportation, etc.) at an average rate of 11 kW. Although sunlight varies with season and time of day, solar energy falls on the United States at an average intensity of 200 W/m2 . Assuming you live in an average location, what total solar-cell area would you need to provide all of your energy needs with energy from the sun?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:02

Problem 37

LASIK eye surgery uses pulses of laser light to shave off tissue from the cornea, reshaping it. A typical LASIK laser emits a 1.0-mm-diameter laser beam with a wavelength of 193 nm. Each laser pulse lasts 15 ns and contains 1.0 mJ of light energy.
a. What is the power of one laser pulse?
b. During the very brief time of the pulse, what is the intensity
of the light wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:05

Problem 38

Using a dish-shaped mirror, a solar cooker concentrates the sun’s energy onto a pot for cooking. A cooker with a 1.5-m-diameter dish focuses the sun’s energy onto a pot with a diameter of 25 cm. Given that the intensity of sunlight is about 1000 W/m2
a. How much solar power does the dish capture?
b. What is the intensity at the base of the pot?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:54

Problem 39

The world's most powerful laser is the LFEX laser in Japan. It can produce a 2 petawatt $\left(2 \times 10^{15} \mathrm{W}\right)$ laser pulse that last for 1 ps. The laser is focused onto a small spot that is $30 \mu \mathrm{m}$ in diameter. What is the light intensity within this spot?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:42

Problem 40

What is the sound intensity level of a sound with an intensity of $3.0 \times 10^{-6} \mathrm{W} / \mathrm{m}^{2}$ ?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:23

Problem 41

What is the sound intensity of a whisper at a distance of 2.0 m,
in W/m2? What is the corresponding sound intensity level in dB?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:53

Problem 42

The record for the world’s loudest burp is 109.9 dB, measured at a distance of 2.5 m from the burper. Assuming that this sound was emitted as a spherical wave, what was the power emitted by the burper during his record burp?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:17

Problem 43

The sound intensity from a jack hammer breaking concrete is 2.0 W/m2 at a distance of 2.0 m from the point of impact. This is sufficiently loud to cause permanent hearing damage if the operator doesn’t wear ear protection. What are (a) the sound intensity and (b) the sound intensity level for a person watching from 50 m away?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:46

Problem 44

A concert loudspeaker suspended high off the ground emits 35 W of sound power. A small microphone with a 1.0 cm2 area is 50 m from the speaker. What are (a) the sound intensity and (b) the sound intensity level at the position of the microphone?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:11

Problem 45

II The African cicada is the world's loudest insect, producing a sound intensity level of $107 \mathrm{dB}$ at a distance of $0.50 \mathrm{m} .$ What is the intensity of its sound (in $\mathrm{W} / \mathrm{m}^{2}$ ) as heard by someone standing $3.0 \mathrm{m}$ away?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
04:43

Problem 46

From a distance of 4.0 m, a bystander listens to a jackhammer breaking concrete. How far would he need to move from the jackhammer so that its perceived loudness decreases by a factor of 8?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:21

Problem 47

A rock band playing an outdoor concert produces sound at 120 dB 5.0 m away from their single working loudspeaker. What is the sound intensity level 35 m from the speaker?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:40

Problem 48

Your ears are sensitive to differences in pitch, but they are not very sensitive to differences in intensity. You are not capable of detecting a difference in sound intensity level of less than 1 dB. By what factor does the sound intensity increase if the sound intensity level increases from 60 dB to 61 dB?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:46

Problem 49

30 seconds of exposure to 115 dB sound can damage your hearing, but a much quieter 94 dB may begin to cause damage after 1 hour of continuous exposure. You are going to an outdoor concert, and you’ll be standing near a speaker that emits 50 W of acoustic power as a spherical wave. What minimum
distance should you be from the speaker to keep the sound intensity level below 94 dB?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:44

Problem 50

When you speak, your voice sounds 10 dB louder to someone standing directly in front of you than to someone at the same distance but directly behind you. What is the ratio of the intensity of your voice for someone in front of you to the intensity for someone behind you?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:12

Problem 51

An opera singer in a convertible sings a note at 600 Hz while cruising down the highway at 90 km/h. What is the frequency heard by
a. A person standing beside the road in front of the car?
b. A person standing beside the road behind the car?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:46

Problem 52

An osprey’s call is a distinct whistle at 2200 Hz. An osprey calls while diving at you, to drive you away from her nest. You hear the call at 2300 Hz. How fast is the osprey approaching?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:51

Problem 53

A whistle you use to call your hunting dog has a frequency of 21 kHz, but your dog is ignoring it. You suspect the whistle may not be working, but you can’t hear sounds above 20 kHz. To test it, you ask a friend to blow the whistle, then you hop on your bicycle. In which direction should you ride (toward or away from your friend) and at what minimum speed to know if the whistle is working?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:09

Problem 54

An echocardiogram uses 4.4 MHz ultrasound to measure blood flow in the aorta. The blood is moving away from the probe at 1.4 m/s. What is the frequency shift of the reflected ultrasound?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:43

Problem 55

A friend of yours is loudly singing a single note at 400 Hz while driving toward you at 25.0 m/s on a day when the speed of sound is 340 m/s.
a. What frequency do you hear?
b. What frequency does your friend hear if you suddenly start
singing at 400 Hz?

Nishant Kumar
Nishant Kumar
Numerade Educator
02:25

Problem 56

The frequency of light emitted from hydrogen present in the Andromeda galaxy has been found to be 0.10% higher than that from hydrogen measured on earth. Is this galaxy approaching or receding from the earth, and at what speed?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:13

Problem 57

A Doppler blood flow unit emits ultrasound at 5.0 MHz. What is the frequency shift of the ultrasound reflected from blood moving in an artery at a speed of 0.20 m/s?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
04:33

Problem 58

A train whistle is heard at 300 Hz as the train approaches town. The train cuts its speed in half as it nears the station, and the sound of the whistle is then 290 Hz. What is the speed of the train before and after slowing down?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:01

Problem 59

At the 18 km cruising altitude of Concorde, a passenger aircraft that flew at twice the speed of sound, the temperature was -57°C. What was the Concorde’s cruising speed?

Vipender Yadav
Vipender Yadav
Numerade Educator
02:07

Problem 60

A 2.0-m-long string is under 20 N of tension. A pulse travels the length of the string in 50 ms. What is the mass of the string?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:43

Problem 61

A female orb spider has a mass of 0.50 g. She is suspended from a tree branch by a 1.1 m length of 0.0020-mm-diameter silk. Spider silk has a density of 1300 kg/m3 . If you tap the branch and send a vibration down the thread, how long does it take to reach the spider?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:43

Problem 62

A spider spins a web with silk threads of density 1300 kg/m3 and diameter 3.0 mm. A typical tension in the radial threads of such a web is 7.0 mN. Suppose a fly hits this web. Which will reach the spider first: the very slight sound of the impact or the disturbance traveling along the radial thread of the web?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:26

Problem 63

In 2003, an earthquake in Japan generated 1.1 Hz waves that traveled outward at 7.0 km/s. 200 km to the west, seismic instruments recorded a maximum acceleration of 0.25g along the east-west axis.
a. How much time elapsed between the earthquake and the first
detection of the waves?
b. Was this a transverse or a longitudinal wave?
c. What was the wavelength?
d. What was the maximum horizontal displacement of the
ground as the wave passed?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:11

Problem 64

A coyote can locate a sound source with good accuracy by
comparing the arrival times of a sound wave at its two ears.
Suppose a coyote is listening to a bird whistling at 1000 Hz.
The bird is 3.0 m away, directly in front of the coyote’s right
ear. The coyote’s ears are 15 cm apart.
a. What is the difference in the arrival times of the sound at the
left ear and the right ear?
b. What is the ratio of this times difference to the period of the
sound wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:59

Problem 65

A wave travels along a steel string with a speed of 22 m/s. At some point along the string, its diameter doubles. What is the speed of the wave in this thicker part?

Susan Hallstrom
Susan Hallstrom
Numerade Educator
01:13

Problem 66

The string in Figure P15.66 has a linear density of $7.2 \times 10^{-5} \mathrm{kg} / \mathrm{m} .$ What is the speed of a wave on this string?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
04:35

Problem 67

Low-frequency vertical oscillations are one possible cause of motion sickness, with 0.30 Hz having the strongest effect. Your boat is bobbing in place at just the right frequency to cause you the maximum discomfort. The water wave that is bobbing the boat has crests that are 30 m apart.
a. What is the speed of the waves?
b. What will be the boat’s vertical oscillation frequency if you drive
the boat at 5.0 m/s in the direction of the oncoming waves?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:10

Problem 68

Figure P15.68 shows two snapshot graphs taken 10 ms apart, with the blue curve being the first snapshot. What are the (a) wavelength, (b) speed, (c) frequency, and (d) amplitude of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:46

Problem 69

The pressure in a sound wave in steel is given by $p(x, t)=p_{\text {atm }}+p_{0} \cos \left(2.4 x-\left(1.4 \times 10^{4}\right) t\right),$ where $p_{\text {atm }}$ is atmospheric pressure, $p_{0}$ is the amplitude of the wave, $x$ is in $\mathrm{m}$, and $t$ in s. What are the speed and frequency of this wave?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:08

Problem 70

A wave on a string is described by $y(x, t)=(3.0 \mathrm{cm}) \times$ $\cos [2 \pi(x /(2.4 \mathrm{m})+t /(0.20 \mathrm{s}))] .$ where $x$ is in $\mathrm{m}$ and $t$ in $\mathrm{s}$
a. In what direction is this wave traveling?
b. What are the wave speed, frequency, and wavelength?
c. At $t=0.50 \mathrm{s},$ what is the displacement of the string at $x=0.20 \mathrm{m} ?$

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:14

Problem 71

Write the y-equation for a wave traveling in the negative x-direction with wavelength 50 cm, speed 4.0 m/s, and amplitude 5.0 cm

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:27

Problem 72

A point on a string undergoes simple harmonic motion as a sinusoidal wave passes. When a sinusoidal wave with speed 24 m/s, wavelength 30 cm, and amplitude of 1.0 cm passes, what is the maximum speed of a point on the string?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:01

Problem 73

The threshold of hearing—the lowest-intensity sound a person can hear—depends on, among many factors, the frequency of the sound. During a hearing exam, the hearing specialist creates an audiogram, a graph of the patient’s hearing threshold (in dB) versus sound frequency; thresholds above 20 dB indicate hearing loss. For the audiogram shown in Figure P15.73, what is the ratio of the sound intensity 1in W/m2 2 of the faintest sound the
patient can hear at 1 kHz to that of the faintest sound she can hear at 6 kHz?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:01

Problem 74

The total power consumption by all humans on earth is approximately 1013 W. Let’s compare this to the power of incoming solar radiation. The intensity of radiation from the sun at the top of the atmosphere is 1380 W/m2. The earth’s radius is 6.37 * 106 m.
a. What is the total solar power received by the earth?
b. By what factor does this exceed the total human power
consumption?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:44

Problem 75

A dark blue cylindrical bottle is 22 cm high and has a diameter of 7.0 cm. It is filled with water. The bottle absorbs 60% of the light that shines on it as it lies on its side in the noonday sun, with intensity 1000 W/m2 By how much will the temperature of the water increase in 5.0 min if there’s negligible
heat loss to the surrounding air?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:57

Problem 76

Assume that the opening of the ear canal has a diameter of 7.0 mm. For this problem, you can ignore any focusing of energy into the opening by the pinna, the external folds of the ear.
a. How much sound power is “captured” by one ear at 0 dB,
the threshold of hearing?
b. How much energy does one ear “capture” during 1 hour of
listening to a lecture delivered at a conversational volume of
60 dB?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:08

Problem 77

The sound intensity $50 \mathrm{m}$ from a wailing tornado siren is $0.10 \mathrm{W} / \mathrm{m}^{2} .$ What is the sound intensity level $300 \mathrm{m}$ from the siren?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:06

Problem 78

One of the loudest sound generators ever created is the Danley Sound Labs Matterhorn. When run at full power, the device uses an input power of 40,000 W. The output power registers 94 dB at 250 m. What is the efficiency of this device— that is, the ratio of output power to input power?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:19

Problem 79

A harvest mouse can detect sounds below the threshold of human hearing, as quiet as -10 dB. Suppose you are sitting in a field on a very quiet day while a harvest mouse sits nearby. A very gentle breeze causes a leaf 1.5 m from your head to rustle, generating a faint sound right at the limit of your ability to hear it. The sound of the rustling leaf is also right at the threshold of hearing of the harvest mouse. How far is the harvest mouse from the leaf?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:50

Problem 80

A speaker at an open-air concert emits 600 W of sound power, radiated equally in all directions.
a. What is the intensity of the sound 5.0 m from the speaker?
b. What sound intensity level would you experience there if
you did not have any protection for your ears?
c. Earplugs you can buy in the drugstore have a noise reduction
rating of 23 decibels. If you are wearing those earplugs but
your friend Phil is not, how far from the speaker should Phil
stand to experience the same loudness as you?

Sheh Lit Chang
Sheh Lit Chang
University of Washington
02:40

Problem 81

A physics professor demonstrates the Doppler effect by tying a 600 Hz sound generator to a 1.0-m-long rope and whirling it around her head in a horizontal circle at 100 rpm. What are the highest and lowest frequencies heard by a student in the classroom? Assume the room temperature is 20°C.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
03:17

Problem 82

When the heart pumps blood into the aorta, the pressure
gradient—the difference between the blood pressure inside
the heart and the blood pressure in the artery—is an important
diagnostic measurement. A direct measurement of the pressure
gradient is difficult, but an indirect determination can be made
by inferring the pressure difference from a measurement of
velocity. Blood is essentially at rest in the heart; when it leaves
and enters the aorta, it speeds up significantly and—according
to Bernoulli’s equation—the pressure must decrease. A doctor
using 2.5 MHz ultrasound measures a 6000 Hz frequency shift
as the ultrasound reflects from blood ejected from the heart.
a. What is the speed of the blood in the aorta?
b. What is the difference in blood pressure between the inside
of the heart and the aorta? Assume that the patient is lying
down and that there is no difference in height as the blood
moves from the heart into the aorta.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:55

Problem 83

As discussed in the chapter, many species of bats find flying insects by emitting pulses of ultrasound and listening for the reflections. This technique is called echolocation. Bats possess several adaptations that allow them to echolocate very effectively. 83. | Although we can’t hear them, the ultrasonic pulses are very loud. In order not to be deafened by the sound they emit, bats can temporarily turn off their hearing. Muscles in the ear cause
the bones in their middle ear to separate slightly, so that they don’t transmit vibrations to the inner ear. After an ultrasound pulse ends, a bat can hear an echo from an object a minimum of 1 m away. Approximately how much time after a pulse is emitted is the bat ready to hear its echo?
A. 0.5 ms B. 1 ms C. 3 ms D. 6 ms

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:20

Problem 84

Bats are sensitive to very small changes in frequency of the reflected waves. What information does this allow them to determine about their prey?
A. Size B. Speed C. Distance D. Species

Sheh Lit Chang
Sheh Lit Chang
University of Washington
01:16

Problem 85

Some bats have specially shaped noses that focus ultrasound echolocation pulses in the forward direction. Why is this useful?
A. Increasing intensity reduces the time delay for a reflected
pulse.
B. The energy of the pulse is concentrated in a smaller area,
so the intensity is larger; reflected pulses will have a larger
intensity as well.
C. Increasing intensity allows the bat to use a lower frequency
and still have the same spatial resolution.

Sheh Lit Chang
Sheh Lit Chang
University of Washington
00:59

Problem 86

Some bats utilize a sound pulse with a rapidly decreasing frequency. A decreasing-frequency pulse has
A. Decreasing wavelength.
B. Decreasing speed.
C. Increasing wavelength.
D. Increasing speed.

Sheh Lit Chang
Sheh Lit Chang
University of Washington