A car is moving at 20ms-¹ along a straight road and sounds its horn at 500 Hz. What frequency do you hear aa the car is approaching you? (Use speed of sound = 340ms-¹)
Answer: 531 Hz
Explanation
The frequency heard by the observer as the car approaches is given by the Doppler effect formula:
f' = f * (v / (v - v_s))
where f = 500 Hz, v = 340 m/s, and v_s = 20 m/s.
f' = 500 * (340 / (340 - 20))
= 500 * (340 / 320)
= 500 * 1.0625
= 531.25 Hz
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