Question

A man standing on a train hears the train whistle. A moment later he hears an answering whistle from an approaching train. Why does the whistle he hears from the approaching train change pitch?

• A. The frequency of the waves of the approaching train's whistle is decreasing
• B. The frequency of the waves of the approaching train's whistle is increasing
• C. The loudness of the waves of the approaching train is decreasing
• D. The loudness of the waves of the approaching train is increasing
• E. The echoes of the two trains' whistles are combining

Answer 1

The correct option is B The frequency of the waves of the approaching train's whistle is increasing

By Doppler's effect ,

If the train on which man is standing is stationary , then the frequency of approaching train's whistle heard by man will be ,

$f^{\prime }=f\left(\frac{v}{v-v_s}\right)$

it is clear that apparent frequency $f^{\prime }$ heard by man is greater than true frequency $f$.

If the train on which man is standing is moving toward the approaching train , then the frequency of approaching train's whistle heard by man will be ,

$f^{\prime }=f\left(\frac{v+v_o}{v-v_s}\right)$

it is clear that apparent frequency $f^{\prime }$ heard by man is greater than true frequency $f$.

in both the conditions frequency of the waves of the approaching train's whistle is increasing.