Final answer:
The stationary person experiences a high frequency as the train approaches and a lower frequency as it moves away due to the Doppler Effect. The engineer on the moving train hears a constant frequency as there's no relative motion between them and the sound source.
Step-by-step explanation:
Understanding the Doppler Effect in Railway Acoustics
When considering the scenario of a stationary person at the side of the tracks and a moving train, we are dealing with a phenomenon known as the Doppler Effect. This physical phenomenon concerns the change in frequency or wavelength of a wave in relation to an observer moving relative to the wave source.
(a) A stationary person at the side of the tracks will observe two different frequencies. As the train approaches, the frequency of the sound will be higher due to the waves being compressed; this is often perceived as a higher-pitched sound. As the train passes and moves away, the frequency decreases because the sound waves are stretched, resulting in a lower pitch.
(b) The train's engineer, who is in motion with the train, will experience a constant frequency. This is because the source of the sound (the train's whistle or horn) is moving at the same speed as the observer (the engineer), so there is no relative change in the distance between them. Therefore, the pitch of the sound remains the same for the engineer inside the moving train.