You are driving along a highway at 35.0 m/s when you hear the siren of a police car approaching you from behind at constant speed and you perceive the frequency as 1310 Hz. You are relieved
That he is in pursuit of a different driver when he continues past you, but now you perceive the
Frequency as 1240 Hz. What is the frequency of the siren in the police car? The speed of sound in
Air is 343 m/s.
A) 1270 Hz
B) 1300 Hz
C) 1320 Hz
D) 1360 Hz
E) 1370 Hz

Question

Quizplus · Accepted Answer

The correct answer is A.The Doppler effect is a change in the frequency of a wave in relation to an observer who is moving relative to the wave source. When the observer is moving towards the wave source, the frequency is increased, and when the observer is moving away from the wave source, the frequency is decreased.In this case, the observer (you) is moving away from the wave source (the police car), so the frequency you perceive is decreased. The original frequency of the siren is the average of the two frequencies you perceive, which is (1310 Hz + 1240 Hz) / 2 = 1275 Hz. However, the speed of sound in air is 343 m/s, and you are moving at 35.0 m/s, so the actual frequency of the siren is 1275 Hz + (35.0 m/s / 343 m/s) * 1275 Hz = 1270 Hz.

You Are Driving Along a Highway at 35