Question

An engine is approaching a cliff at a constant speed. When it is at a distance of 0.9 km from cliff it sounds a whistle. The echo of the sound is heard by the driver after 5 seconds. Velocity of sound in air is equal to 330 ms^-1. The speed of the engine is ______ km/h.

पर्याय

• 108

• 105

• 100

• 102

Answer 1

An engine is approaching a cliff at a constant speed. When it is at a distance of 0.9 km from cliff it sounds a whistle. The echo of the sound is heard by the driver after 5 seconds. Velocity of sound in air is equal to 330 ms^-1. The speed of the engine is 108 km/h.

Explanation:

Let speed of engine be u m/s.

Distance covered by sound in time 5 seconds.

= (0.9 × 10³) + (0.9 × 10³ – 5u)

speed of sound = 330 m/s

∴ Time = `"distance"/"speed"`

= `(1.8 xx 10^3 - 5u)/330`

`5 = (1.8 xx 10^3 - 5u)/330`

5u = 1.8 × 10³ - 330 × 5

u = `(1.8 xx 10^3)/5 - 330`

= 30 m/s

= `30 xx 18/5` km/hr

= 108 km/hr