A-Level Physics AQA 7408

Concept of moment of inertia

$I = mr^2$ for a point mass.

$I = Σmr^2$ for an extended object.

Qualitative knowledge of the factors that affect the moment of inertia of a rotating object.

Expressions for moment of inertia will be given where necessary.

Rotational kinetic energy

$E_k = \dfrac{1}{2}Iω^2$

Factors affecting the energy storage capacity of a flywheel.

Use of flywheels in machines.

Use of flywheels for smoothing torque and speed, and for storing energy in vehicles, and in machines used for production processes.

Rotational motion

Angular displacement, angular speed, angular velocity, angular acceleration, $ω = \dfrac{∆θ}{∆t}$, $α = \dfrac{∆ω}{∆t}$

Representation by graphical methods of uniform and non-uniform angular acceleration.

Equations for uniform angular acceleration;

$ω_2 = ω_1 + αt$, $θ = \Big(\dfrac{ω_1+ω_2}{2}\Big)t$

$θ = ω_1t + \dfrac{αt^2}{2}$, $ω_2^2 = ω_1^2 + 2αθ$

Students should be aware of the analogy between rotational and translational dynamics.

Torque and angular acceleration

$T = Fr$

$T = Iα$

Angular momentum

$\text{angular momentum} = Iω$

Conservation of angular momentum.

Angular impulse = change in angular momentum; $T∆t=∆(Iω)$ where T is constant.

Applications may include examples from sport.

Work and power

$W = Tθ$; $P = Tω$

Awareness that frictional torque has to be taken into account in rotating machinery.