Question

A car moving with a speed of 72 kmph towards a hill. Car blows horn at a distance of 1800 m from the hill. If echo is heard after 10s, the speed of sound (in m/s) is

• A. 300
• B. 320
• C. 340
• D. 360

Answer 1

The correct option is C 340

$\vartheta =72km/hr$
$=72\times \frac{1000}{3600}m/s$
$=20m/s$

Distance travelled by sound waves for echo is:
$2\times 1800-\vartheta$ (time of echo) $=$ speed of sound $\times$ time of echo
$3600-20\left(10\right)=V_s\times 10$
$3400=V_s\times 10$

$\Rightarrow V_s=340m/s$