An engine approaches a hill with a constant speed. When it is at … - Vidyadip

MCQ

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 .... $m/s$

A
$32$
B
$27.5$
C
$60$
✓
$30$

✓

Answer

Correct option: D.

$30$

d
Let after 5 sec engine at point $C$

$t=\frac{A B}{330}+\frac{B C}{330}$

$5=\frac{0.9 \times 1000}{330}+\frac{\mathrm{BC}}{330}$

$\therefore \mathrm{BC}=750 \mathrm{m}$

Distance travelled by engine in $5\, sec$

$=900 \mathrm{m}-750 \mathrm{m}=150 \mathrm{m}$

Therefore velocity of engine

$=\frac{150 \mathrm{m}}{5 \mathrm{sec}}=30 \mathrm{m} / \mathrm{s}$

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