Question

A person in an open car is approaching a vertical mountain with uniform velocity 90 km/h. The person blows horn, then hears its echo after a time 4 s. The distance of the car from the mountain when the horn was blown is. (Velocity of sound in air is 335 ms${}^{-1}$)

• A. 1440m
• B. 1240 m
• C. 720m
• D. 620m

Answer 1

Answer: (C)

720m

$\vartheta =90km/h$

$=90\times \frac{1000}{3600}m/s$

$=25m/s$

So, distance travelled by sound waves for echo is:

$2x-\vartheta$ (time of echo) $=$ speed of sound $\times$ time of echo

$2x-25\left(4\right)=335\times 4$

$2x=1440$

$x=720m$