3.6.1.4
Free oscillations have no energy input; forced oscillations are driven externally
Forced Vibrations & Resonance — AQA A-Level Physics
Key Definition
Free oscillation — An oscillation where there are only internal forces acting and no energy is transferred to or from the system. The system oscillates at its natural frequency.
Key Definition
Forced oscillation — An oscillation maintained by a periodic external driving force that transfers energy into the system. The system oscillates at the driving frequency.
- A free oscillationAn oscillation that occurs at the natural frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). of a system with no external driving force or energyThe capacity to do work. Measured in joules (J). input after the initial displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).. always oscillates at the natural frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz).The frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). at which a system oscillates freely when displaced from equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. and released, with no external driving force. f_0.
- A forced oscillationAn oscillation that occurs when an external periodic driving force is applied to a system. oscillates at the driving frequency f (which may differ from f_0).
- Example of free: striking a tuning fork.
- Example of forced: pushing a child on a swing at regular intervals.
Worked Example
Classify: (i) striking a tuning fork, (ii) a glass shattering from a high-pitched sound, (iii) playing a clarinet.
Show Solution
1
Tuning fork
Free vibration. Once struck, it vibrates at its natural frequencyThe frequency at which a system oscillates freely when displaced from equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. and released, with no external driving force. with no external forces.
2
Glass shattering
Forced vibration. The sound wave is the external driving force, forcing the glass to vibrate at the sound's frequency until it breaks.
3
Clarinet
Forced vibration. The player's breath sustains vibrations in the air column at the same frequency as the driving force from the reed.
Answer
(i) Free, (ii) Forced, (iii) Forced.
Examiner Tips and Tricks
Never write 'a free oscillationAn oscillation that occurs at the natural frequency of a system with no external driving force or energyThe capacity to do work. Measured in joules (J). input after the initial displacementThe distance moved in a particular direction from a starting point. A vector quantity. Measured in metres (m).. is not forced.' Mark schemes want you to refer to internal vs external forces and whether energyThe capacity to do work. Measured in joules (J). is transferred into the system.