A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n^{'})$of the sound heard by the observer is plotted against time $(t)$, which of the following best represents the resulting curve

Question

Dr. Abubacker · Accepted Answer

(d) It is known that Doppler’s effect depends on velocity not on distance. When the source is approaching the stationary observer, the apparent frequency heard by the observer is
$n^{'} = \frac{v 	imes n}{v - v_{s}} = constant$
But $n^{'} > n$.
When the source has crossed the observer, apparent frequency heard by the observer is
$n^{''} = \frac{v 	imes n}{v + v_{s}} = another\ constant$
But $n^{''} < n$.option (d) is correct.

Q. A railway engine whistling at a constant frequency moves with a constant speed. It goes past a stationary observer standing beside the railway track. The frequency $(n^{'})$of the sound heard by the observer is plotted against time $(t)$, which of the following best represents the resulting curve

Explanation

Reviewed by Dr. Abubacker

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