Question

The pitch of whistle of an engine appears to drop by 30% of the original value when it passes a stationary observer. If speed of sound in air is 350 m/s, then the speed of engine in m/s is ______.

विकल्प

• 87.5

• 105

• 150

• 175

Answer 1

The pitch of whistle of an engine appears to drop by 30% of the original value when it passes a stationary observer. If speed of sound in air is 350 m/s, then the speed of engine in m/s is 150.

Explanation:

Given: Speed of sound (v) = 350 m/s

n' = 0.7 n

Also,

n' = `n(v/(v + v_s))`

0.7 n = `n(350/(350 + v_s))`

0.7 = `350/(350 + v_s)`

350 + v_s = `350/0.7`

350 + v_s = 500

v_s = 500 − 350

= 150 m/s