Paddle Balls and Frequencies
The paddle ball on its rubber band moves in response to the finger supporting it. If the finger moves with the natural frequency f0 of the ball on the rubber band, then a resonance is achieved, and the amplitude of the ball's oscillations increases dramatically. At higher and lower driving frequencies, energy is transferred to the ball less efficiently, and it responds with lower-amplitude oscillations.
Source
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"OpenStax College, Forced Oscillations and Resonance. February 23, 2013."
http://cnx.org/content/m42247/latest/
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