Question

Two cars are moving on two perpendicular roads towards a crossing with uniform speeds of 72 km/hr and 36 km/hr. If first car blows horn of frequency 280 Hz, then the frequency of horn heard by the driver of second car when line joining the cars make ${45}^{\circ }$ angle with the roads; will be

• A. 321 Hz
• B. 298 Hz
• C. 289 Hz
• D. 280 Hz

Answer 1

The correct option is B

298 Hz

Here $v_4=72km/hr=20m/sec$

$V_B$ = 36 km/hr = 10 m /sec

n' = n $\left(\frac{v+v_{B}cos{45}^{\circ }}{v-v_{A}cos{45}^{\circ }}\right)$

= n' = 280 $\left(\frac{340+\frac{10}{\sqrt{2}}}{340-\frac{20}{\sqrt{2}}}\right)=298$ Hz