OCR · A-Level · Physics
Course version: H556 · Version from Sept 2015
Damping: (a) free and forced oscillations
A free oscillation occurs when a system oscillates at its natural frequency with no external periodic driving force, after being displaced and released.
Checked against the OCR H556 specification (opens in a new tab) on .
- Point 1 of 5
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A free oscillation occurs when a system oscillates at its natural frequency with no external periodic driving force, after being displaced and released. Its amplitude stays constant only if damping is negligible; otherwise it decays. A forced oscillation occurs when a periodic external driving force is applied, so the system oscillates at the driving frequency rather than its natural frequency. The amplitude of a forced oscillation depends on how close the driving frequency is to the natural frequency. When they are equal, resonance occurs and amplitude reaches a maximum. Damping reduces the peak amplitude at resonance and broadens the resonance curve, shifting the peak slightly.
What earns the marks
- A free oscillation has no external periodic driving force and occurs at the natural frequency of the system.
- A forced oscillation is driven by a periodic external force and oscillates at the driving frequency.
- Amplitude of a forced oscillation depends on the frequency difference between driving and natural frequencies.
- Resonance occurs when driving frequency equals natural frequency, giving maximum amplitude.
- Damping reduces the maximum amplitude at resonance and broadens the resonance curve.
- Examples include a pendulum swinging freely and a mass on a spring driven by a vibrating support.
Common mistakes and how to put them right
Thinking a forced oscillator always vibrates at its natural frequency.
Put it right: It vibrates at the driving frequency.
Believing resonance gives infinite amplitude in every real system.
Put it right: Damping limits the maximum amplitude.
Confusing free oscillation with zero damping.
Put it right: A free oscillation can still be damped and decay in amplitude.
Assuming damping increases the resonant amplitude.
Put it right: Damping reduces it and widens the response curve.
Exam tips
- State clearly whether a driving force is present before classifying an oscillation as free or forced.
- Link resonance to the condition driving frequency equals natural frequency, and mention damping when discussing amplitude.
- Use a resonance curve to show how damping changes peak height and width.
Try the lesson
The start of the Damping: (a) free and forced oscillations lesson, made from checked teaching notes for this specification point.
Listen
A free oscillation occurs when a system oscillates at its natural frequency with no external periodic driving force, after being displaced and released. Its amplitude stays constant only if damping is negligible; otherwise it decays. A forced oscillation occurs when a periodic external driving force is applied, so the system oscillates at the driving frequency rather than its natural frequency. The amplitude of a forced oscillation depends on how close the driving frequency is to the natural frequency. When they are equal, resonance occurs and amplitude reaches a maximum. Damping reduces the peak amplitude at resonance and broadens the resonance curve, shifting the peak slightly. A strong answer earns marks for these points. A free oscillation has no external periodic driving force and occurs at the natural frequency of the system. A forced oscillation is driven by a periodic external force and oscillates at the driving frequency. Amplitude of a forced oscillation depends on the frequency difference between driving and natural frequencies. Resonance occurs when driving frequency equals natural frequency, giving maximum amplitude. Exam tip: State clearly whether a driving force is present before classifying an oscillation as free or forced.
See it
Key words
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Free oscillation
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Forced oscillation
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Resonance
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See every definition
- Free oscillation
- Occurs when a system oscillates at its natural frequency with no external periodic driving force, after being displaced and released.
- Forced oscillation
- Occurs when a periodic external driving force is applied, so the system oscillates at the driving frequency rather than its natural frequency.
- Resonance
- Occurs when the driving frequency equals the natural frequency, and amplitude reaches a maximum.
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