Q: A tsunami travels across the open ocean with an average depth of 4000 m. What is the approximate speed of the tsunami? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

$$v = \sqrt{g d} = \sqrt{9.8 \times 4000} \approx 198\ \mathrm{m/s}$$

Q: A spider web thread has a linear density of $$1.0 \times 10$$^{-5}$$\ \mathrm{kg/m}$$ and is under a tension of $$0.15\ \mathrm{N}. $$What is the speed of a wave traveling along the thread?

$$v = \sqrt{\frac{T_s}{\mu}} = \sqrt{\frac{0.15}{1.0 \times 10$$^{-5}$$}} = 120\ \mathrm{m/s}$$

Q: A sound wave travels through air at $$20^\circC$$ ($$T = 293 K) $$with a molar mass $$M = 0.029\ \mathrm{kg/mol}$$ and $$\gamma = 1.4. $$What is the speed of sound in air? (Use $$R = 8.31\ \mathrm{J/(mol\cdot K)})$$

$$v = \sqrt{\frac{\gamma R T}{M}} = \sqrt{\frac{1.4 \times 8.31 \times 293}{0.029}} \approx 343\ \mathrm{m/s}$$

Q: A 1.0 mW laser pointer emits light in a 1.0-mm-diameter beam. What is the intensity of the laser beam? (Recall $$I = \frac{P}{A}$$, area of a circle $$A = \pi r^2$)

$$I = \frac{1.0 \times 10$$^{-3}$$\ \mathrm{W}}{\pi (0.5 \times 10$$^{-3}$$\ \mathrm{m})^2$} \approx 1.3 \times 10^3$\ \mathrm{W/m^2$}$$

Question 1

Which of the following best describes the difference between a transverse wave and a longitudinal wave?

Accepted Answer

In a transverse wave, particles move perpendicular to the direction of wave travel, while in a longitudinal wave, particles move parallel to the direction of wave travel.

Question 2

A tsunami travels across the open ocean with an average depth of 4000 m. What is the approximate speed of the tsunami? (Use $$g = 9.8\ \mathrm{m/s^2$})$$

Accepted Answer

$$v = \sqrt{g d} = \sqrt{9.8 	imes 4000} \approx 198\ \mathrm{m/s}$$

Question 3

Which of the following statements about mechanical and electromagnetic waves is correct?

Accepted Answer

Mechanical waves require a material medium to travel, while electromagnetic waves do not.

Question 4

A spider web thread has a linear density of $$1.0 	imes 10$$^{-5}$$\ \mathrm{kg/m}$$ and is under a tension of $$0.15\ \mathrm{N}. $$What is the speed of a wave traveling along the thread?

Accepted Answer

$$v = \sqrt{\frac{T_s}{\mu}} = \sqrt{\frac{0.15}{1.0 	imes 10$$^{-5}$$}} = 120\ \mathrm{m/s}$$

Question 5

Which property of a wave is determined by the properties of the medium and not by the frequency of the source?

Accepted Answer

Wave speed

Question 6

A sound wave travels through air at $$20^\circC$$ ($$T = 293 K) $$with a molar mass $$M = 0.029\ \mathrm{kg/mol}$$ and $$\gamma = 1.4. $$What is the speed of sound in air? (Use $$R = 8.31\ \mathrm{J/(mol\cdot K)})$$

Accepted Answer

$$v = \sqrt{\frac{\gamma R T}{M}} = \sqrt{\frac{1.4 	imes 8.31 	imes 293}{0.029}} \approx 343\ \mathrm{m/s}$$

Question 7

A sinusoidal wave is described by $$y(x, t) = A \sin(kx - \omega t + \$$phi_0$$). $$What does the parameter $$k$$ represent?

Accepted Answer

The wave number, $$k = \frac{2\pi}{\lambda}$$, with units of rad/m

Question 8

A 1.0 mW laser pointer emits light in a 1.0-mm-diameter beam. What is the intensity of the laser beam? (Recall $$I = \frac{P}{A}$$, area of a circle $$A = \pi r^2$)

Accepted Answer

$$I = \frac{1.0 	imes 10$$^{-3}$$\ \mathrm{W}}{\pi (0.5 	imes 10$$^{-3}$$\ \mathrm{m})^2$} \approx 1.3 	imes 10^3$\ \mathrm{W/m^2$}$$

Question 9

Which of the following is true about the Doppler effect for sound waves?

Accepted Answer

The observed frequency increases if the source approaches the observer.

Question 10

A police siren emits a frequency of 550 Hz as it approaches and 450 Hz as it recedes. If the speed of sound is 343 m/s, what is the speed of the police car?

Accepted Answer

$$v_s = \frac{f_+ - f_-}{f_+ + f_-} v = \frac{550 - 450}{550 + 450} 	imes 343 \approx 34.3\ \mathrm{m/s}$$

Question 11

Which of the following best explains why a tsunami slows down as it approaches the shore?

Accepted Answer

The speed of a shallow-water wave is proportional to the square root of the ocean depth, so as depth decreases, speed decreases.

Question 12

A sinusoidal wave has a frequency of 100 Hz and travels at 200 m/s. What is its wavelength?

Accepted Answer

$$\lambda = \frac{v}{f} = \frac{200}{100} = 2.0\ \mathrm{m}$$

Question 13

Which of the following is NOT a property of sound waves in air?

Accepted Answer

The speed of sound depends on the pressure of the air.

Question 14

A wave on a string is described by $$y(x, t) = 0.05 \sin(4\pi x - 200\pi t)$$, where $$x$$ and $$y$$ are in meters and $$t in $$seconds. What is the wave speed?

Accepted Answer

$$v = \frac{\omega}{k} = \frac{200\pi}{4\pi} = 50\ \mathrm{m/s}$$

Question 15

Which of the following best describes the difference between a snapshot graph and a history graph for a wave?

Accepted Answer

A snapshot graph shows displacement as a function of position at a single instant, while a history graph shows displacement as a function of time at a single position.

Question 16

A sound wave has an intensity of $$1.0 	imes 10$$^{-6}$$\ \mathrm{W/m^2$}. $$What is the sound intensity level in decibels? (Use $$I_0 = 1.0$$ 	imes $$10^{-12}$$\ \mathrm{$$W/m^2$$}$$)

Accepted Answer

$$\beta = 10 \log_{10}\left(\frac{I}{I_0$}ight) = 10 \log_{10}\left(\frac{1.0 	imes 10$$^{-6}$$}{1.0 	imes 10$$^{-12}$$}ight) = 60\ \mathrm{dB}$$

Question 17

Which of the following is a correct statement about the energy transfer in a wave?

Accepted Answer

A wave transfers energy through the medium, but does not transfer matter.

Question 18

A light wave travels from air (index of refraction $$n_1 = 1.0$) into glass (n_2$ = 1.5). If $$the speed of light in air is $$c = 3.00 	imes 10^8$\ \mathrm{m/s}$$, what is the speed of light in glass?

Accepted Answer

$$v = \frac{c}{n} = \frac{3.00 	imes 10^8$}{1.5} = 2.00 	imes 10^8$\ \mathrm{m/s}$$

Traveling Waves and Sound: Physics Study Notes

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