Use of flywheels for smoothing torque and speed, and for storing energy in…

The idea

A flywheel is a rotating mass that stores rotational kinetic energy, Eₖ = ½Iω². Because its angular momentum resists rapid change, it smooths torque and speed: when a machine's driving torque fluctuates, the flywheel absorbs energy during surplus and releases it during deficit, so output speed varies less. In vehicles, a flywheel can store energy recovered during braking and return it during acceleration, reducing fuel use; in production machines such as presses, it supplies a large short burst of energy from a smaller continuous motor. Its effectiveness depends on moment of inertia I and angular speed ω, so mass distribution and safe maximum speed matter.

What earns the marks

• A flywheel stores rotational kinetic energy given by Eₖ = ½Iω², so energy storage increases with moment of inertia and with the square of angular speed.
• Smoothing occurs because the flywheel's large moment of inertia resists changes in angular velocity; it absorbs energy when the driving torque exceeds the load and releases it when the load exceeds the drive.
• In a vehicle, a flywheel can store energy during braking and return it during acceleration, reducing the demand on the engine and improving efficiency.
• In production machinery such as a punch press, the flywheel supplies a large energy burst for each stroke while the motor provides a smaller, steadier average power.
• The moment of inertia depends on mass and how that mass is distributed about the axis, so a rim-weighted flywheel stores more energy for the same total mass.
• Practical limits include material strength at high angular speed, bearing friction and the need to keep angular speed within a safe range.

Common mistakes and how to put them right

• Treating a flywheel as a source of energy rather than a store.

  Put it right: It stores rotational kinetic energy and must be spun up by an external torque before it can return energy.

• Assuming doubling angular speed doubles stored energy.

  Put it right: Eₖ ∝ ω², so doubling ω quadruples the stored energy for unchanged I.

• Confusing torque smoothing with speed smoothing.

  Put it right: The flywheel reduces fluctuations in both by exchanging energy, but it does not eliminate the average torque or power requirement.

Exam tips

• Link each context to the same physics: identify where energy enters the flywheel and where it leaves, then relate this to ½Iω².
• When comparing flywheels, state clearly whether I, ω or both change, and calculate the resulting energy ratio rather than guessing.
• Use correct units and notation, for example J for energy, rad s⁻¹ for angular speed and kg m² for moment of inertia.