OCR A Level Physics

Revision Notes

5.6.5 Examples of Forced Oscillations & Resonance

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Examples of Forced Oscillations & Resonance

  • Resonance occurs for any forced oscillation where the frequency of the driving force is equal to the natural frequency of the oscillator
    • For example, a glass smashing from a high pitched sound wave at the right frequency
  • Some other practical examples of forced oscillations and resonance include:
    • An organ pipe
    • Radio receivers
    • Microwave oven
    • Magnetic resonance imaging (MRI)
  • In an organ pipe
    • Air molecules vibrate in an air column setting up a stationary wave in the pipe
    • This causes the air molecules to resonate leading to an increase in amplitude of sound

Stationary Wave Organ Resonance, downloadable AS & A Level Physics revision notes

Standing waves forming inside an organ pipe from resonance

  • Radio receivers
    • The radio is “tuned” by setting its natural frequency equal to that of a radio station
    • The radio tuned so that the electric circuit resonates at the same frequency as the specific broadcast
    • The resonance of the radio waves allows the signal to be amplified by the receiver to listen

  • Microwave oven
    • Conventional cooking methods involve transferring heat energy by conduction or convection
    • A microwave transfers heat energy by radiation i.e. microwaves of a particular frequency that resonate with the water molecules in food


  • Magnetic resonance imaging (MRI)
    • This type of scanner is a widely used medical diagnostic tool used to look at organs and structures inside the body
    • The atomic nuclei in the body are made to resonate with incoming radio waves (of the order of 100 MHz)
    • The signals are then sent to a computer to create digital scans and provide a detailed image of the scanned area

Barton's Pendulums

  • A mechanical system commonly used to show resonance is Barton's pendulums
  • A set of light pendulums labelled A-E are suspended from a string
    • A heavy pendulum X, with a length L, is attached to the string at one end and will act as the driving pendulum

  • When pendulum X is released, it pushes the string and begins to drive the other pendulums
  • Most of the pendulums swing with a low amplitude but pendulum C with the same length has the largest amplitude
    • This is because its natural frequency is equal to the frequency of pendulum X (the driving frequency)

Bartons Pendulums, downloadable AS & A Level Physics revision notes

Barton's pendulums helps display resonance

  • The phase of the oscillations relative to the driver are:
    • Pendulums E and D with lengths < L are in phase
    • Pendulum C with length = L is 0.5π out of phase
    • Pendulums B and A with lengths > L are π out of phase

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