Question

The apparent frequency of the whistle of an engine changes in the ratio 9:8 as the engine passes a stationary observer. If the velocity of the sound is 340 ms^-1, then the velocity of the engine is:

• A. 20 m/s
• B. 180 m/s
• C. 40 m/s
• D. 340 m/s

Answer 1

The correct option is A

20 m/s

When the engine approaches the observer, the apparent frequency is v/(v-v_S) and when it moves away from the observer, the apparent frequency is v/(v+v_S). The frequency therefore changes in the ratio (v+v_S)/(v-v_S). We have therefore, (v+v_S) / (v-v_S) = 9/8 from which the velocity of the source, v_S = v/17 = 340/17 = 20m/s.