A body with that is not accelerating is in equilibrium. A body travelling at constant speed is in translational equilibrium. A body rotating at constant angular velocity is in rotational equilibrium.

Key Concepts

A body is in *rotational equilibrium* if the sum of the anticlockwise torques is equal to the sum of the clockwise torques about a pivot:

\(\sum\Gamma_\textrm{anticlockwise}= \sum\Gamma_\textrm{clockwise}\)

A body is in *translational equilibrium *if no resultant force acts:

\(\sum F_\textrm{upward}=\sum F_\textrm{downward}\)

\(\sum F_\textrm{left}=\sum F_\textrm{right}\)

Like torque for force and moment of inertia for mass, there is also a rotational equivalent for acceleration, *angular acceleration.*

Angular acceleration is defined as the rate of change of angular velocity:

\(a={\Delta \omega\over \Delta t}={{\omega _f-\omega_i}\over t}\)

- \(\alpha\) is angular acceleration (rads
^{-2}) - \(\omega_f\) is final angular velocity (rads
^{-1}) - \(\omega_i\) is initial angular velocity (rads
^{-1}) - \(t\) is the time taken (s)

We can convert between tangential and angular acceleration:

\(a=r\alpha\)

- \(a\) is tangential acceleration (ms
^{-2}) - \(r\) is the radius of the circle (m)
- \(\alpha\) is angular acceleration (rads
^{-2})

Recall from Oscillations and Circular motion, there are two methods for finding angular velocity:

- Angular velocity is the instantaneous rate of change of angular displacement (rad): \(\omega={\Delta \theta \over \Delta t}\)
- For repeated circular motion, angular velocity is the total angle moved through for a complete circle (\(2\pi\) rad) divided by the time period (s): \(\omega = {2\pi \over T}=2\pi f\), where \(f\) is the frequency (Hz).

Note the three forms of acceleration:

- Angular acceleration is the rate of change of angular velocity, or the rate of change in the angle subtended.
- Angular acceleration is is proportional to the tangential acceleration, the rate of change of translational velocity. This requires a resultant force acting along a tangent to the circle.
- These are not related to centripetal acceleration. Centripetal acceleration is towards the centre of the circle and thus requires a resultant force along a radius to the circle, perpendicular to the tangent.