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# A car moves with a speed of 54 km h-1 towards a cliff.

Question:

A car moves with a speed of $54 \mathrm{~km} \mathrm{~h}^{-1}$ towards a cliff. The horn of the car emits sound of frequency $400 \mathrm{~Hz}$ at a speed of $335 \mathrm{~m} \mathrm{~s}^{-1}$.

(a) Find the wavelength of the sound emitted by the horn in front of the car.

(b) Find the wavelength of the wave reflected from the cliff.

(c) What frequency does a person sitting in the car hear for the reflected sound wave?

(d) How many beats does he hear in 10 seconds between the sound coming directly from the horn and that coming after the reflection?

Solution: