As shown if Fig. a vibrating tuning fork of frequency 512 Hz is moving towards the wall with a speed `2(m)/(s)`. Take speed of sound as `v=340(m)/(s)` and answer the following questions.
Q. If the listener, along with the source, is moving towards the wall with the same speed i.e., `2(m)/(s)`, such that the source remains between the listerner and the wall, number of beats heard by the listerner per second will be

Options

A. 4
B. 8
C. 0
D. 6

Answer

B

Explanation

As no relative motion is there between observer and listener, hence frequency heard by observer is 512 Hz. He will observe frequency reflected from wall, `f_1=515Hz`. Hence, the wave reflected from wall will act as another source of frequency 515 Hz.
Therefore, the frequency received by the observer from wall is
`f_2=(v+v_0)/(v)f_1`
`=(342)/(340)xx515=518Hz`
Hence, beats observed is `f_2f=518-512=6`.