Q: The speed of sound in air at 20°C is 343 m/s. If you hear thunder 8.0 seconds after seeing lightning, how far away did the lightning strike?

$$343 \times 8.0 = 2744\ \text{m}$$

Q: Microwaves in a microwave oven have a wavelength of 12 cm. What is their frequency? (Speed of light $$c = 3.0 \times 10^8$\ \text{m/s})$$

$$f = \frac{3.0 \times 10^8$}{0.12} = 2.5 \times 10^9$\ \text{Hz}$$

Q: A laser pointer emits 1.0 mW of light power into a beam that is 1.0 mm in diameter. What is the intensity of the laser beam?

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

Q: A person shouts with a power of 1.0 W. What is the sound intensity level at 1.0 m from the person? (Threshold of hearing $$I_0 = 1.0$$ \times $$10^{-12}$$\ \text{W/m}^2$$)

$$L = 10 \log_{10}\left(\frac{I}{I_0$}\right) = 10 \log_{10}\left(\frac{1.0}{1.0 \times 10$$^{-12}$$}\right) = 120\ \text{dB}$$

Q: A radio wave has a frequency of 5.8 GHz. What is its wavelength? (Speed of light $$c = 3.0 \times 10^8$\ \text{m/s})$$

$$\lambda = \frac{c}{f} = \frac{3.0 \times 10^8$}{5.8 \times 10^9$} = 0.052\ \text{m}$$

Question 1

Which of the following best describes a transverse wave?

Accepted Answer

The particles in the medium move perpendicular to the direction in which the wave travels.

Question 2

A sound wave is produced by a loudspeaker. Which statement is true about the motion of air molecules as the sound wave passes?

Accepted Answer

Air molecules oscillate parallel to the direction of the wave.

Question 3

The speed of sound in air at 20°C is 343 m/s. If you hear thunder 8.0 seconds after seeing lightning, how far away did the lightning strike?

Accepted Answer

$$343 	imes 8.0 = 2744\ 	ext{m}$$

Question 4

Which property of the medium determines the speed of a mechanical wave?

Accepted Answer

The properties of the medium, such as temperature and molecular mass.

Question 5

A sinusoidal wave is described by $$y(x, t) = A \cos\left(2\pi\left(\frac{x}{\lambda} - \frac{t}{T}\right)\right). $$What does $$A$$ represent?

Accepted Answer

The amplitude of the wave.

Question 6

A boat moves to the right at 5.0 m/s and encounters ocean waves moving to the left with a wavelength of 110 m and period of 8.3 s. What is the vertical displacement of the boat after 20 s if it started at a crest?

Accepted Answer

$$y_f = 1.0 \cos\left(2\pi\left(\frac{100}{110} + \frac{20}{8.3}\right)\right)$$

Question 7

What is the relationship between wave speed $$v$$, frequency $$f$$, and wavelength $$\lambda$$?

Accepted Answer

$$v = f \lambda$$

Question 8

Microwaves in a microwave oven have a wavelength of 12 cm. What is their frequency? (Speed of light $$c = 3.0 	imes 10^8$\ 	ext{m/s})$$

Accepted Answer

$$f = \frac{3.0 	imes 10^8$}{0.12} = 2.5 	imes 10^9$\ 	ext{Hz}$$

Question 9

A laser pointer emits 1.0 mW of light power into a beam that is 1.0 mm in diameter. What is the intensity of the laser beam?

Accepted Answer

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

Question 10

The intensity of a spherical wave at distance $$r$$ from the source is given by:

Accepted Answer

$$I = \frac{P}{4\pi r^2$}$$

Question 11

A person shouts with a power of 1.0 W. What is the sound intensity level at 1.0 m from the person? (Threshold of hearing $$I_0 = 1.0$$ 	imes $$10^{-12}$$\ 	ext{W/m}^2$$)

Accepted Answer

$$L = 10 \log_{10}\left(\frac{I}{I_0$}ight) = 10 \log_{10}\left(\frac{1.0}{1.0 	imes 10$$^{-12}$$}ight) = 120\ 	ext{dB}$$

Question 12

A radio wave has a frequency of 5.8 GHz. What is its wavelength? (Speed of light $$c = 3.0 	imes 10^8$\ 	ext{m/s})$$

Accepted Answer

$$\lambda = \frac{c}{f} = \frac{3.0 	imes 10^8$}{5.8 	imes 10^9$} = 0.052\ 	ext{m}$$

Question 13

Which of the following best describes the Doppler effect?

Accepted Answer

A change in frequency due to the motion of the source or observer.

Question 14

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

Accepted Answer

$$v_s = \frac{(f_+ - f_-)}{(f_+ + f_-)} 	imes v = \frac{(550 - 450)}{(550 + 450)} 	imes 343 = 34.3\ 	ext{m/s}$$

Question 15

If a source of light is moving away from you, what happens to the wavelength you detect?

Accepted Answer

The wavelength is longer and the light is red shifted.

Question 16

A boat is at rest in the ocean. The wave has amplitude $$A = 1.0\ 	ext{m}$$, wavelength $$\lambda = 190\ 	ext{m}$$, and speed $$v = 14\ 	ext{m/s}. $$What is the maximum vertical speed of the boat?

Accepted Answer

$$v_{max} = 2\pi f A = 0.48\ 	ext{m/s}$$

Question 17

Which of the following is true about electromagnetic waves?

Accepted Answer

They travel at the speed of light in a vacuum.

Chapter 15: Traveling Waves and Sound - Study Notes

Study Guide - Practice Questions

Study Guide - Flashcards