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9745 H2 PHYSICS (2008) 8. Oscillations Damped and forced oscillations: resonance (k) describe graphically how the amplitude of a forced oscillation changes with frequency near to the natural frequency of the system, and understand qualitatively the factors which determine the frequency response and sharpness of the resonance.

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For a forced oscillation, when the driving frequency is equal to the natural frequency of the oscillating system, maximum energy is transferred from the periodic force (driver) to the oscillating system which will vibrate with maximum amplitude. This phenomenon is called resonance.

Frequency Response Graph (under different degrees of damping)
The Effect of Damping on Forced Oscillations
When damping is present, it reduces the amplitude (y-axis) of the forced oscillation for all driver frequencies (x-axis) and it causes the maximum amplitude to be reached when the driving frequency is a little less than the natural frequency.
This reduction in amplitude is more significant at the resonance frequency and frequencies close to it, so that we say damping reduces the sharpness of resonance -- that is, the oscillating system responses little (small change in the amplitude) over a wide range of frequencies.
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