BITSAT 2015 Paper

Let v = speed of sound and vS = speed of tuning forks. Apparent frequency of fork moving towards the observer is
n1 = (

v

v−vs

) n
Apparent frequency of the fork movingaway from the observer is
n2 = (

v

v+vs

) n
If f is the number of beats heard per second. then f = n1–n2
⇒ f = (

v

v−vs

) n - (

v

v+vs

) n
⇒ f =

v(v+vs)−v(v−vs)

v2−vs2

(n)
⇒ 2vvsnv2−vs2 = f
⇒ 2 (

vs

v

) = n = f
{if vs ≪ v}
⇒ v5 =

fv

2n

putting v = 340 m/s, f = 3, n = 340 Hz we get,
vs =

340×3

3×340

= 1.5 m/s