Rotational Kinetic Energy
Physics: AQA A Level, topic 11.1.2
Energy of a Flywheel
A flywheel is a revolving wheel used in a machine to accumulate and store energy.
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:
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.
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.
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.
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.
Test yourself on Rotational Kinetic Energy
What is the maximum angular velocity of a flywheel limited by?
Which of these is NOT a potential use of flywheels?