Physics
When two similar sources vibrate with slightly different frequencies, a regular rise and fall occurs in the loudness of tone. This phenomenon is known as

Echo
Doppler effect
Resonance
Beats

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Physics
A metal ball and a rubber ball, both having the same mass, strike a wall normally with the same velocity. The rubber ball rebounds and the metal ball does not rebound. It can be concluded that

The initial momentum of the rubber ball is greater than that of the metal ball
Both suffer the same change in momentum
The metal ball suffers greater change in momentum
The rubber ball suffers greater change in momentum

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