Question

Two trains are moving towards each other with speeds of 20 m/s and 15 m/s relative to the ground. The first train sounds a whistle of frequency 600 Hz, the frequency of the whistle heard by a passenger in the second train before the train meeets is (the speed of sound in air is 340 m/s)

• A. 600 Hz
• B. 585 Hz
• C. 645 Hz
• D. 666 Hz

Answer 1

Answer: (D)

666 Hz

Answer 2

The perceived frequency (f')
\hspace20mm f' = f_0 \bigg( \frac{v + v_0}{v - v_s}\bigg)
Given, $v = 340\, m/s, v_0 = 15\, m/s$ and $v_s = 20 m/s$
\therefore \hspace20mm f' = 600 \times \bigg( \frac{340+ 15}{340 -20}\bigg)
\hspace30mm =\frac{355}{320} \times 600\, Hz