A level Physics · Edexcel 9PH0 · Topic 13B
Damping and resonance
Key points
- Free oscillations happen at the system's natural frequency with no external force after release.
- Damping is the loss of energy to the surroundings, often through friction or air resistance, so the amplitude decreases.
- Light damping: amplitude falls gradually over many cycles. Critical damping: returns to equilibrium in the shortest time without oscillating (e.g. car suspension). Heavy damping: returns slowly with no oscillation.
- Forced oscillations: a periodic driving force makes the system oscillate at the driving frequency.
- Resonance occurs when the driving frequency equals the natural frequency: energy transfer is most efficient and amplitude is greatest.
- More damping lowers and broadens the resonance peak and shifts it slightly to lower frequency.
- Examples: useful in musical instruments and MRI; harmful for bridges and buildings, so damping is added to reduce it.
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Real oscillations lose energy and die away, and a regular push can make them grow. Damping and resonance explain why a swing builds up when pushed in time and why bridges and buildings are designed to avoid it.
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