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 seconds. If the speed of sound in air is 330 m/s, then the speed of the engine is :

Explanation

The situation is shown in figure-6.9.
If the speed of the engine is v, the distance travelled by the engine in 5 s. will be 5 v.And hence, the distance travelled by soundin reachingthe hill and comingbacktothe movingdriver is given as
`S = 900 + (900 - 5v) = (1800 - 5v)`
Thus time interval between the original sound and its echo is
`t = (S)/(v_(sound)`
`5 = (1800 - 5v)/(330)`
On solving we get v = 30m/s