A source emitting 320 Hz moves directly towards a stationary wall at 20 m/s. The speed of sound is 340 m/s. The source compares the echo with its own emitted sound. If the beat frequency is N Hz, find N.
Physics · Wave Motion · doppler effect
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Question

A source emitting 320 Hz moves directly towards a stationary wall at 20 m/s. The speed of sound is 340 m/s. The source compares the echo with its own emitted sound. If the beat frequency is N Hz, find N.

Physics · Wave Motion · doppler effect

G1 Physics · Accepted Answer

The echo frequency heard by the moving source is fₑ = f×(v+u)/(v−u). Substitute: fₑ = 320(340+20)/(340−20) = 320×360/320 = 360 Hz. The emitted frequency is 320 Hz. Hence, fᵦ = 360−320 = 40 Hz. Therefore, N = 40.