A car fitted with a device which transmits sound 60 times per minute. There is no wind and speed of sound in still air is 345 m/s. If you hear the sound 68 times per minute when you are moving towards the car with a speed of 12 m/s, the speed of the car must be nearly.

Options

A. 20.0 m/s towards
B. 30.0 m/s towards you
C. 10.0 m/s away from you
D. 10.0 m/s towards you

Answer

B

Explanation

`C = 345 m//s" "f_(ob) = 68 "time/1 min"`

`f'=(C+12)/(C-V_(2))f_(0)`
`rArr (68)/(60)=(345+12)/(345-V_(2))`
`345-V_(2)=357 xx(60)/(68)`
`V_(2)=345-(357xx60)/(68)=345-315=30 m//s`.