devarshi-dt-logo

Question:

A toy-car, blowing its horn, is moving with a steady speed of 5 m/s, away from a wall. An observer, towards whom the toy car is moving, is able to hear 5 beats per second. If the velocity of sound in air is 340 m/s, the frequency of the horn of the toy car is close to: 680 Hz, 510 Hz, 340 Hz, 170 Hz

680 Hz

340 Hz

510 Hz

170 Hz

Solution:

The correct option is D 170 Hz
Frequency heard by the observer directly from source
f1=f(340/(340-5))
Frequency heard by the observer coming after reflection from wall
f2=f(340/(340+5))
Beats=(f1-f2)
5=f(340/(340-5) - 340/(340+5)) → f=170Hz