Wave Mechanics Question 683
Question: A man is watching two trains, one leaving and the other coming in with equal speeds of 4m/ sec. If they sound their whistles, each of frequency 240 Hz, the number of beats heard by the man (velocity of sound in air = 320 m/sec) will be equal to
Options:
A) 6
B) 3
C) 0
D) 12
Show Answer
Answer:
Correct Answer: A
Solution:
[a] Frequency of sound heard by the man from approaching train. $ n _{a}=n( \frac{v}{v-v _{s}} )=240( \frac{320}{320-4} )=243,Hz $ Frequency of sound heard by the man from receding train $ n _{r}=n( \frac{v}{v+v _{s}} ) $
$ =240( \frac{320}{320+4} )=237,Hz $ Hence, number of beats heard by man per sec $ =n _{a}-n _{r}=243-237=6 $
BETA
