# ch16 - CHAPTER 16 WAVES AND SOUND ANSWERS TO FOCUS ON...

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CHAPTER 16 WAVES AND SOUND ANSWERS TO FOCUS ON CONCEPTS QUESTIONS 1. (c) It is indeed the direction in which the disturbance occurs that distinguishes the two types of waves. 2. (a) As one domino topples against the next one in line, it is moving partly perpendicular and partly parallel to the direction along which the disturbance propagates. 3. (d) The amplitude specifies the maximum excursion of the spot from the spot’s undisturbed position, and the spot moves through this distance four times during each cycle. For instance, in one cycle starting from its undisturbed position, the spot moves upward a distance A , downward a distance A (returning to its undisturbed position), downward again a distance A , and finally upward again a distance A (returning to its undisturbed position). 4. (d) According to Equation 16.1, the speed v of the wave is related to the wavelength λ and the frequency f by v = f λ . The speed depends only on the properties of the string (tension, mass, and length) and remains constant as the frequency is doubled. According to Equation 16.1, then, the wavelength must be cut in half. 5. 0.80 m 6. (b) The amplitude specifies the maximum excursion of a particle from its undisturbed position and has nothing to do with the wave speed. The amplitude does affect the speed of the simple harmonic motion, however. Equation 10.8 indicates that v max is proportional to the amplitude. 7. (c) As discussed in Section 16.3, the speed of the wave is greater when the tension in the rope is greater, which it is in the upper portion of the rope. The rope has a mass m and, hence, a weight. The upper portion of the rope has a greater tension than does the lower portion, because the upper portion supports the weight of a greater length of rope hanging below. 8. (e) According to Equation 16.2, the speed is proportional to the square root of the tension. Since the tension in string 1 is three times that in string 2, the speed in string 1 is 3 1.73 = times the speed in string 2. 9. 0.0039 kg/m 10. 161 m/s

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65 WAVES AND SOUND 11. (e) Condensations are regions where the air pressure is increased above the normal pressure, and rarefactions are regions where the air pressure is decreased below the normal pressure. When the amplitude decreases to zero, the pressure of the air is no longer being increased above or being decreased below the normal air pressure. Therefore, there are no longer any condensations or rarefactions. 12. (b) Sound travels faster in liquids than in gases. The greater speed in water ensures that the echo will return more quickly in water than in air. 13. (d) The frequency of the sound is determined by the vibrating diaphragm of the horn. The sound wave travels through the air and contacts the surface of the water, where it causes the water molecules to vibrate at the same frequency as the molecules in the air. However, the speed of sound in air is smaller than in air. According to Equation 16.1, the wavelength is proportional to the speed, when the frequency is constant. As a result, the wavelength is smaller in the air than in the water.
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