Angular acceleration is the rate of change of angular velocity. In three dimensions, it is a pseudovector. In SI units, it is measured in radians per second squared (rad/s2), and is usually denoted by the Greek letter alpha (α).
The angular acceleration vector is defined as:
where is the angular velocity vector.
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The angular acceleration of a point particle α can be connected to the applied torque τ by the following equation:
where m is the mass of the particle and I is its moment of inertia.
Above relationship indicates that, unlike the relationship between force and acceleration, the angular acceleration need not be directly proportional or even parallel to the torque. In fact, this is true whenever the moment of inertia of the particle changes with time.