Question

The apperent frequency of the whistle of an engine changes by the ratio 5/3 as the engine passes a stationary observer. If the velocity of sound is 340 m/s, then the velocity of the engine is –

• A. 340 m/s
• B. 170 m/s
• C. 85 m/s
• D. 42.5 m/s

Answer 1

Answer: (C)

85 m/s

Answer 2

$\frac { \mathrm { n } ^ { \prime } } { \mathrm { n } ^ { \prime \prime } }$= $\frac { V + V _ { s } } { V - V _ { s } }$ = $\frac { 5 } { 3 }$

3V + 3V_s = 5V – 5V_s

V_s = $\frac { V } { 4 }$ = $\frac { 340 } { 4 }$ = 85 m/s