SAT Subject Physics Practice Question 499: Answer and Explanation

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Question: 499

15. A driver in an automobile hears a siren behind her and pulls over to let a fire engine pass. The sound of the siren changes as the fire truck approaches, is beside her, and passes her. Which of the following is/are true under these conditions?

I. The pitch of the sound increases as the fire truck approaches.

II. The wavelength of the sound increases as the fire truck approaches.

III. The wavelength of the sound increases after the fire truck passes.

A. I only
B. II only
C. I and III only
D. II and III only
E. I, II, and III

Correct Answer: C

Explanation:

The correct answer is (C). The listener hears the sound waves being bunched together as the siren approaches him, causing the pitch of the sound he hears to increase. When the fire truck passes the listener, the sound waves seem to be spread apart as the pitch of the sound decreases due to the lengthening of the sound waves.

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