A train engine sounds a whistle at frequency . The frequency heard by a passenger is

Step-by-step solution

The passenger and the train engine are moving with the same velocity, so there is no relative motion between the source of sound (engine) and the observer (passenger). According to the Doppler effect, the apparent frequency is given by f' = ( v - v_o v - v_s ) . Since the velocity of the observer v_o is equal to the velocity of the source v_s , we get f' = . Thus, the frequency heard by the passenger is equal to .