Question

The engine of a train sounds a whistle at frequency v. The frequency heard by a passenger is
(a) $>\mathrm{v}$
(b) $<\mathrm{v}$
(c) $=\frac{1}{\mathrm{v}}$
(d) $=\mathrm{v}$

Answer 1

(d) $=\mathrm{v}$

For the Doppler effect to occur, there must be relative motion between the source and the observer. However, this is not the case here. Hence, the frequency heard by the passenger is υ.