Question

The frequency of echo will be ______ Hz if the train blowing a whistle of frequency 320 Hz is moving with a velocity of 36 km/h towards a hill from which an echo is heard by the train driver. The velocity of sound in air is 330 m/s.

विकल्प

• 300

• 320

• 340

• 320

Answer 1

The frequency of echo will be 340 Hz if the train blowing a whistle of frequency 320 Hz is moving with a velocity of 36 km/h towards a hill from which an echo is heard by the train driver. The velocity of sound in air is 330 m/s.

Explanation:

From Doppler's effect, the frequency of the car w.r.t. the hill

f₁ = `("v"/("v" - "v"_"s"))"f" = (330/320)320 = 330` Hz

∴ Frequency of the sound reflected by hill w.r.t. the car i.e., echo heard by the observer,

f₂ = `(("v" + "v"_0)/"v")"f"_1 = ((330 + 10)/330) xx 330 = 340` Hz