Question

A police car is travelling at a speed of 144km/h. It has a siren with a frequency of 1120Hz. Assume that the speed of sound in air is 320m/s.

a) If the car is moving toward you, what frequency will you hear for the siren?

b) If the car had been moving away from you at the same speed, what frequency would you have heard?

Answer 1

Here, the pdice car is the source cohich is moring with a speod V_s = 144 km/h = 144 times 5/18 m /s = 40 m /s (a) As the car more towards you(the observer) , the frquency deterted by the observer is f = v/v -v_s times f_r: v is the velocity of sound and is source frequency v = 320 m/s f_0 = 1120 Hz (b) If the car moves away from the observer, the frequency of the sirren heard by the observer is f = v /v+ v_s f_0 = 320/320 + 40 times 1120 = 995.55 Hz