Question

find the word that matches the definition: the tendency of sound from a moving object to change frequency relative to a stationary observer

Explanation:

The Doppler effect is a phenomenon where the frequency of a wave (such as sound) changes for an observer when the source of the wave is moving relative to the observer. For example, when an ambulance siren approaches, the pitch (frequency) seems higher, and as it moves away, the pitch seems lower. This matches the given definition about the tendency of sound frequency from a moving object to change relative to a stationary observer.

Answer:

Doppler effect