Q. An engine approaches a hill with a constant speed. When it is at a distance of $0.9\ km$ it blows a whistle, whose echo is heard by the driver after $5\ sec.$ If speed of sound in air is $330\ m/s,$ the speed of engine is
A) $10\ m/s$
B) $20\ m/s$
C) $30\ m/s$
D) $40\ m/s$
Explanation
(c) If the speed of engine is $v,$ the distance traveled by engine in $5\ sec$ will be $5v,$ and hence the distance traveled 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
$t = \frac{(1800 - 5v)}{330} = 5 \Rightarrow v = 30\ m/s$
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.