Free & Forced Oscillations (AQA A Level Physics): Revision Note
Exam code: 7408
Free & forced oscillations
Free oscillations
Free oscillations occur when there is no transfer of energy to or from the surroundings
This happens when an oscillating system is displaced and then left to oscillate
In practice, this only happens in a vacuum
However, anything vibrating in air is still considered a free vibration as long as there are no external forces acting upon it
Therefore, a free oscillation is defined as:
An oscillation where there are only internal forces (and no external forces) acting and there is no energy input
A free vibration always oscillates at its resonant frequency
Forced oscillations
In order to sustain oscillations in a simple harmonic system, a periodic force must be applied to replace the energy lost in damping
This periodic force does work against the resistive force responsible for decreasing the oscillations
It is sometimes known as an external driving force
These are known as forced oscillations (or vibrations), and are defined as:
Oscillations acted on by a periodic external force where energy is given in order to sustain oscillations
Forced oscillations are made to oscillate at the same frequency as the oscillator creating the external, periodic driving force
For example, when a child is on a swing, they will be pushed at one end after each cycle in order to keep swinging and prevent air resistance from damping the oscillations
These extra pushes are the forced oscillations
Without them, the child will eventually come to a stop
Worked Example
State whether the following are free or forced oscillations:
(i) Striking a tuning fork [1]
(ii) Breaking a glass from a high pitched sound [1]
(iii) The interior of a car vibrating when travelling at a high speed [1]
(iv) Playing the clarinet [1]
Answer:
(i) Striking a tuning fork
When a tuning fork is struck, it will vibrate at its natural frequency and there are no other external forces
This is a free vibration [1 mark]
(ii) Breaking a glass from a high pitched sound
The glass is forced to vibrate at the same frequency as the sound until it breaks
The frequency of the high-pitched sound is the external driving frequency
This is a forced vibration [1 mark]
(iii) The interior of a car vibrating when travelling at a particular speed
The interior of the car vibrates at the same frequency as the wheels travelling over a rough surface at a high speed
This is a forced vibration [1 mark]
(iv) Playing the clarinet
The air from the player's lungs is used to sustain the vibration in the air column in a clarinet to create and hold a sound
The air column inside the clarinet mimics the vibrations at the same frequency as the air forced into the mouthpiece of the clarinet (the reed)
This is a forced vibration [1 mark]
Examiner Tips and Tricks
Avoid writing 'a free oscillation is not forced to oscillate'. Mark schemes are mainly looking for a reference to internal and external forces and energy transfers
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