Question

the change in pitch of a trains horn as it passes while youre standing still can be explained by
a) ultrasonic vibrations.
b) the doppler effect.
c) resonance.
d) sympathetic vibration.

Explanation:

Ultrasonic vibrations are high-frequency sounds beyond human hearing, unrelated to pitch changes from moving sources. Resonance is when an object vibrates at its natural frequency due to external matching vibrations. Sympathetic vibration is a type of resonance where one vibrating object causes another to vibrate. The Doppler effect specifically describes the change in frequency (pitch) of a wave when the source or observer is in relative motion, which matches the scenario of a passing train's horn changing pitch for a stationary observer.

Answer:

B) the Doppler effect.