Question
The frequency of whistle of an engine appears to be (4 / 5)^ th of initial frequency when it crosses a stationary observer. If the velocity of sound is 330 ~m / s , then the speed of engine will be
BITSAT
Physics
Waves and Sound
2021
BITSAT 2021
BITSAT Physics Question (2021) — Solution
Options
- A. 30 ~m / s
- B. 36.6 ~m / s
- C. 40 ~m / s
- D. 330 ~m / s
Answer
B. 36.6 ~m / s
Step-by-step solution
n^ = n v v-v_ s (1) n^ = n v v+v_ s . .(2) From (1) and (2), n^ n^ = v+v_ s v-v_ s . .(3) According to question, n^ n^ = 5 4 v_ s =? v=330 ~m / s .(4) From eq. (3) and (4) 5 4 = [ 330+v_ s 330-v_ s ] 9 v_ s =330 v_ s =36.6 ~m / s
Related: Physics — Waves and Sound · All PYQ Banks