Rotational Kinetic Energy

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Energy of a Flywheel

A flywheel is a revolving wheel used in a machine to accumulate and store energy.

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Rotational kinetic energy

  • The rotational kinetic energy Ek for a body with a moment of inertia I rotating at an angular velocity of ω rad s–1 is given by:
    • Ek=12Iω2E_k = \frac12 I ω^2
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Maximum energy stored

  • The maximum energy storage capacity of a flywheel depends on its mass and shape (hence its moment of inertia) and its maximum angular speed.
  • The maximum allowed angular speed is limited by the breaking stress of the material of which the flywheel is made.
    • For a solid disc of radius R, Ek is proportional to R4 ω2.

Uses of Flywheels

Flywheels are used in machines to accumulate and store energy.

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Regenerative braking

  • In regenerative braking, energy is absorbed by braking and used to ‘charge’ a flywheel, which can accelerate the vehicle at a later time.
    • This is used in Formula 1 racing cars.
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Reduce fluctuations

  • Flywheels can even out fluctuations in rotational speed.
    • E.g. A flywheel is fitted to the crankshaft of an internal combustion or a reciprocating steam engine where the torque provided by the engine varies.
  • The flywheel also takes the engine over the ‘dead centres’.
  • The greater the moment of inertia, the smaller the fluctuation in speed.
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Flywheels in steam engines

  • Flywheels fitted to large Victorian steam engines had a massive rim fitted with spokes.
  • This has a greater moment of inertia than a solid disc flywheel of the same diameter.

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

11Option: Engineering Physics (A2 only)

12Option: Turning Points in Physics (A2 only)

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