Question

A person in a vehicle is moving away from a vertical mountain with uniform velocity. When the vehicle is at a distance 975 m from mountain, the person blows a horn and hears its echo after 6 s. If velocity of sound in air is 340 ms${}^{-1}$, the velocity of the vehicle is :

• A. 36 kmph
• B. 54 kmph
• C. 72 kmph
• D. 90 kmph

Answer 1

The correct option is B 54 kmph

To hear echo distance traveled by second wave is $\mu t+2\times 975=$ speed of sound $\times$ time after which echo is heard

$\mu \left(6\right)+2\times 975=340\times 6$

$6\mu =90$

$\mu =15m/s$

$=\frac{15\times 3600}{1000}km/h$

$=54km/h$