Question

A speeding motorcyclist sees traffic jam ahead of him. He slows down to 36 km/h. He finds that the traffic has eased and a car moving ahead of him at 18 km/h horns at a frequency of 1392 Hz. If the speed of sound is 343 m/s, the frequency of the horn as heard by him will be:

(a) 1332 Hz

(b) 1372 Hz

(c) 1412 Hz

(d) 1454 Hz

Answer 1

Correct option is : (c) 1412 Hz

Here both the observer and the source are moving, we can use the formula of apparent frequency, as

\(f = (\frac{v + v_0}{v + v_s}) = 1392\ (\frac{343 + 10}{343 + 5})\) 

= 1392 \((\frac{353}{348}) = 1412\ Hz\)