PURPOSE: To demonstrate the concept of resonance and the idea of phase shift at resonance.
DESCRIPTION: The mass on the spring has a natural frequency, which can be demonstrated by simply holding one end of the spring a rest and allowing the mass to oscillate freely. Demonstrate resonance as follows: (1) With the mass hanging at rest, move your hand very slowly up and down. The mass follows your hand, showing that the mass and the driving force stay in phase for driving frequencies far below the natural frequency of the oscillator. (2) With the mass hanging at rest, move your hand very rapidly up and down. The mass moves opposite to your hand, showing that the mass and the driving force stay out of phase for driving frequencies far above the natural frequency of the oscillator. (3) Move your hand up and down at the natural frequency of oscillation; the phase relationship for resonance is that motion of the driver (hand) must be 90 degrees ahead of the motion of the oscillator. With an almost imperceptible oscillation of your hand, the resonance condition causes the mass on spring to begin to oscillate with a very large amplitude.
SUGGESTIONS:
REFERENCES: See nice AJP article.
EQUIPMENT: Mass on spring, hand.
SETUP TIME: None.
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