A engine is approaching a hill at 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 s`. If the speed of sound is `340 m//s`, calculate the speed of the engine.

Answer

20 m//s

Explanation

Let `v_e` be the speed of the engine.
distance covered by the engine in `5 s = 5 v_e`
distance covered by sound in reaching the hill and coming back to the moving driver
=`(900 xx 2 - 5 v_e)` `(0.9 km = 900 m)`
=`1800 - 5 v_e`
According to the given conditions, `t = ((1800 - 5 v_e))/(v)`
As `t = 5 s` and `v` (speed of sound) `= 340 m//s`,
`5 = (180 - 5 v_e)/(340)` or `1700 = 1800 - 5 v_e`
or `v_e = 20 m//s`.