CAIE A-Level Physics AS 7.3.1 When a Source of Sound Waves Moves Relative to a Stationary Observer the Questions

A child on a sledge slides down a steep hill and then travels in a straight line up an ice-covered slope, as illustrated in Fig. 3.1.

The sledge passes point A with speed $18 \mathrm{~ms}^{-1}$ at time t=0 and then comes to rest at point B. The child applies a brake to the sledge at point B. The brake does not keep the sledge stationary and it immediately slides back down the slope towards A .

The variation with time t of the velocity v of the sledge from t=0 to $t=24 \mathrm{~s}$ is shown in Fig. 3.2.

The child on the sledge blows a whistle between $t=4.0 \mathrm{~s}$ and $t=8.0 \mathrm{~s}$. The whistle emits sound of frequency 900 Hz . The speed of the sound in the air is $340 \mathrm{~ms}^{-1} . \mathrm{A}$ man standing at point A hears the sound.

Use Fig. 3.2 to

describe and explain qualitatively the variation, if any, in the frequency of the sound heard by the man.