Question

A car is moving with a speed of $72\, km/h$ towards a hill. Car blows horn at a distance of $1800 \,m$ from the hill. If echo is heard after $10\, s$, the speed of sound (in $m/s$) is

• A. 300
• B. 320
• C. 340
• D. 360

Answer 1

Answer: (C)

The speed of the car is $72 \,km/h$
$ = 72 \times \frac{5}{18} = 20 \, m/s$
The distance travelled by car in $10\, s$
$ = 10 \times 20 = 200 \, m $
Hence, the distance travelled by sound in reaching the hill and coming back to the moving driver
$= 1800 + (1800 - 200) $
$= 3400 \, m$
So, the speed of sound $ = \frac{3400}{10} = 340 \, m/s$