Question

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 :

• A. 32 m/s
• B. 27.5 m/s
• C. 60 m/s
• D. 30 m/s

Answer 1

Answer: (D)

30 m/s

Answer 2

Let after 5 sec engine at point C
$t = \frac{AB}{330}+\frac{BC}{330}$
$5 = \frac{0.9\times1000}{330}+\frac{BC}{330}$
$\therefore BC=750\,m$
Distance travelled by engine in 5, sec
$= 900\,m-750\,m= 150\,m$
Therefore velocity of engine
$= \frac{150m}{5sec} = 30 \,m/s$