Edexcel A Level (IAL) Physics-5.27 Free & Forced Oscillations- Study Notes- New Syllabus

Distinction Between Free and Forced Oscillations

Oscillatory motion can be classified as free oscillations or forced oscillations depending on whether an external driving force is applied.

 Free Oscillations

Free oscillations occur when a system is displaced from its equilibrium position and then allowed to oscillate without any external driving force.

• No external periodic force acts on the system.
• The system oscillates at its natural frequency.
• Energy is not supplied after motion begins.

Energy considerations:

• In an ideal system, total mechanical energy is constant.
• In real systems, energy is gradually lost due to damping.

Examples:

• A pendulum released from a small angle.
• A mass on a spring pulled and released.

Forced Oscillations

Forced oscillations occur when an external periodic force continuously supplies energy to the system.

• An external driving force acts on the system.
• The system oscillates at the frequency of the driving force.
• Energy lost due to damping is replaced by the driving force.

Examples:

• A swing being pushed periodically.
• A mass–spring system attached to a vibration driver.

Key Differences Between Free and Forced Oscillations

• Free oscillations require no continuous energy input.
• Forced oscillations require an external energy source.
• Free oscillations occur at natural frequency.
• Forced oscillations occur at driving frequency.

Role of Damping

• In free oscillations, damping causes amplitude to decrease.
• In forced oscillations, damping limits the amplitude.
• Damping is necessary to prevent destructive resonance.

 Link to Resonance

In forced oscillations:

• Resonance occurs when driving frequency equals natural frequency.
• Amplitude becomes maximum.

Important: Resonance does not occur in free oscillations.