Rotational kinetic energy (A-level only) Flashcards – AQA A-level Physics (7408)

What is the equation for the kinetic energy of a body rotating about a fixed axis?

$E_k = \frac{1}{2}I\omega^2$, where $I$ is the moment of inertia about the axis and $\omega$ is angular speed.

What does moment of inertia measure?

How a body's mass is distributed relative to the axis of rotation.

What is the SI unit of energy?

The joule ($\mathrm{J}$).

What is the SI unit of moment of inertia?

$\mathrm{kg\,m^2}$

What is the SI unit of angular speed?

$\mathrm{rad\,s^{-1}}$

In what form does a flywheel store energy?

As rotational kinetic energy.

How does a flywheel’s stored energy depend on its moment of inertia and angular speed?

It increases with the moment of inertia and with the square of the angular speed.

How does mass distribution affect a flywheel’s moment of inertia?

Placing more mass farther from the axis increases its moment of inertia.

Why do material strength and flywheel design affect safe energy storage capacity?

The flywheel must withstand rotational stresses, which limit its maximum safe angular speed.

How does the balance between driving torque and load torque affect a flywheel’s speed and energy exchange?

• Driving torque greater: it speeds up and stores energy.
• Load torque greater: it slows down and returns energy.

What effect does a flywheel’s energy exchange have on a machine’s rotational speed?

It reduces fluctuations in rotational speed.

What effect does a flywheel’s energy exchange have on torque delivery?

It helps smooth torque delivery.

Besides smoothing machine operation, what can a flywheel do in vehicles and production machinery?

Store energy for later use.