Final answer:
A stationary observer will hear a higher frequency as the train approaches and a lower frequency after it passes, due to the Doppler Effect. The train's engineer will hear the true frequency of the whistle without alterations.
Step-by-step explanation:
The question involves understanding Doppler Effect phenomena observed in acoustics and sound frequency alterations due to relative motion, which is a topic covered in high school physics. The frequencies observed by a stationary person as the train approaches will be higher than the actual frequency of the train's whistle due to the Doppler Effect, meaning as the source of the sound (train) comes closer, the sound waves are compressed, leading to a higher pitch. Once the train passes and moves away, the frequency observed will decrease, as the sound waves get stretched, leading to a lower pitch. Conversely, the train's engineer, traveling with the train, will observe the actual frequency of the whistle, as there is no relative motion between the engineer and the whistle.
When a train approaches a stationary person at the side of the tracks, the sound of the train's whistle appears to have a higher frequency. This is because the sound waves are compressed by the train's motion, resulting in a higher pitch. After the train passes, the sound of the whistle appears to have a lower frequency, as the sound waves are stretched out due to the train's movement away from the person.
On the other hand, the train's engineer traveling on the train would perceive a constant frequency of the whistle. This is because the engineer and the whistle are moving at the same velocity, so there is no relative motion between them to affect the frequency of the sound waves.