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 the speed ofsound in air is 330 m/s, calculate the speed of the engine.

Explanation

If the speed ofthe engine is V, the distance travelled by the engine in 5 sec, 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 time interval between the original sound and its echo
`(1800-5V)/330 = t = 5s` (given)
The above equation on solving gives V = 30 m/s.