A speeding motorcyclist sees traffic jam ahead him. He slows down to 36 km hour ⁻¹. He finds that traffic has eased and a car moving ahead of him at 18 km hour ⁻¹ is honking at a frequency of 1392 Hz. If the speed of sound is 343 m s ⁻¹, the frequency of the honk as heard by him will be (a) 1332 Hz (b) 1372 Hz (c) 1412 Hz (d) 1454 Hz

Answer: (c). Motorcyclist (observer) = 36 km/h = 10 m/s towards the car; car (source) = 18 km/h = 5 m/s away from him. f' = f (v + vo)/(v + vs) = 1392 × (343 + 10)/(343 + 5) = 1392 × (353)/(348) f' ≈ 1412 Hz So the answer is (c) 1412 Hz.

A speeding motorcyclist sees traffic jam ahead him. He slows down to… | NEET 2014 (AIPMT)

Q69NEET 2014 · AIPMTMedium

Doppler Effect

A speeding motorcyclist sees traffic jam ahead him. He slows down to 36 km hour ${}^{-1}$ . He finds that traffic has eased and a car moving ahead of him at 18 km hour ${}^{-1}$ is honking at a frequency of 1392 Hz. If the speed of sound is 343 m s ${}^{-1}$ , the frequency of the honk as heard by him will be

Show answer and solution

Answer: (c)

Motorcyclist (observer) $= 36\text{ km/h} = 10\text{ m/s}$ towards the car; car (source) $= 18\text{ km/h} = 5\text{ m/s}$ away from him.
$f' = f\,\dfrac{v + v_o}{v + v_s} = 1392 \times \dfrac{343 + 10}{343 + 5} = 1392 \times \dfrac{353}{348}$
$f' \approx 1412\text{ Hz}$
So the answer is (c) 1412 Hz.

Report a mistake

Doppler Effect question from NEET 2014 (AIPMT)

More questions on Doppler Effect