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

Beats
Doppler effect
Resonance
Echo

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Physics
When a pail of water is swung in a vertical circle, the water does not fall out at the top of the loop when the speed is

Above a certain minimum value depending on amount of water in the pail
Below a certain minimum value depending on amount of water in the pail
Above a certain minimum value irrespective of amount of water in the pail
Below a certain minimum value irrespective of amount of water in the pail

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