Question

A man is standing on a railway platform listening to the whistle of an engine that passes the man at constant speed without stopping. If the engine passes the man at time $t_0$. How does the frequency $f$ of the whistle as heard by the man changes with time

• A.
• B.
• C.
• D. Diagram

Answer 1

The correct option is B

When the train is approaching the stationary observer. The frequency heard by the observer,

$n^{\prime }=\frac{v+v_0}{v}n$

When the train is moving away from the observer. The frequency heard by the observer,

$n^{\prime \prime }=\frac{v-v_0}{v}n$

From those two statements, it is clear that $n^{\prime }$ and $n^{\prime \prime }$ are constant as well as not dependent of time. Thus,

$n^{\prime }>n^{\prime \prime }$

Final Answer: (a)