Question

An engine approaches a hill with a constant speed. When it is at a distance of $0.9km$, it blows a whistle, whose echo is heard by the driver after $5s$. If speed of sound in air is $330m/s$, the speed of engine is

• A. $10m/s$
• B. $20m/s$
• C. $30m/s$
• D. $40m/s$

Answer 1

Answer: (C)

$30m/s$

If the speed of engine is $v$, the distance travelled by engine in $5s$ will be $5v$ and hence the distance travelled by sound in reaching the hill and coming back to the moving driver
$=900+(900-5v)$
$1800-5v$
So, the time interval between original sound and it's echo
$t=\frac{(1800-5v)}{330}=5\Rightarrow v=30m/s$