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 listerner along with the source is moving towards the wall with the same speed i.e., `2(m)/(s)`, such that he (listener) remains between the source and the wall, number of beats heard by him will be

Options

A. 2
B. 6
C. 8
D. 4

Answer

D

Explanation

Frequency received from source directly by observer will remain same. Hence frequency received by observer is `512Hz` let `f_1` be the frequency reflected by wall Then,
`f_1=(v)/(v-v_S)xxf=(340)/(338)xx512=515Hz`
The frequency received by observer (reflected from wall) is
`f_2=((v_v_0)/(v))f=(342)/(340)xx515=518Hz`
Hence beats heard is `f_2-f_1=518-512=6`.