An engine moving towards a wall with a velocity 50 m s - 1 emits a note of 1 . 2 kHz . Speed of sound in air is 350 m s - 1 . The frequency of the note after reflection from the wall as heard by the driver of the engine is

Question

An engine moving towards a wall with a velocity 50 m s - 1 emits a note of 1 . 2 kHz . Speed of sound in air is 350 m s - 1 . The frequency of the note after reflection from the wall as heard by the driver of the engine is, 2 . 4 kHz, 0 . 24 kHz, 1 . 6 kHz, 1 . 2 kHz

MARKS App · Accepted Answer

The reflection sound appears to propagate in a direction opposite to that of moving engine. Thus, the source and the observer can be presumed to approach each other with same velocity. v ′ = v v + v o v - v s = v v + v s v - v s ∵ v o = v s ⟹ v ′ = 1 .2 350 + 50 350 - 50 = 1 .2 × 400 300 = 1 .6 kHz

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