Final answer:
The stationary person observes a higher frequency as the train approaches and a lower frequency after it passes, due to the Doppler Effect, while the train's engineer hears the true frequency of the sound as there is no relative motion between them and the source.
Step-by-step explanation:
Understanding the Observations of Frequencies by Different Observers
The question being asked pertains to how frequency observations differ depending on the position and movement of the observer relative to the source of the sound. This is a classic example of the Doppler Effect in acoustics within the study of Physics.
(a) A stationary person at the side of the tracks will observe a higher frequency as the train approaches them and a lower frequency after the train passes them. This is because the sound waves are compressed as the source approaches the observer, resulting in a higher pitch, and are stretched after the source passes, causing a lower pitch.
(b) The train's engineer traveling on the train will observe the frequency of the sound as it was emitted because for the engineer, the source of the sound is not moving relative to them. This frequency is the actual frequency of the source without any Doppler Effect alterations.
In scenarios such as throwing a ball inside the train, or observing light flashes from within a railcar, the observations made by someone inside the train will differ from someone outside due to relative motion, which is a key concept in both mechanics and relativity.