For rotation about a fixed axis, torque plays the role of force, angular acceleration the role of linear acceleration, and moment of inertia the role of mass.

The rotational equation of motion is

$$\sum\tau=I\alpha.$$

Net torque

Several forces can produce torques about the same axis. Their signed torques add to give the net torque.

A zero net torque means zero angular acceleration for a rigid body with fixed nonzero $I$; the body may still rotate at constant angular velocity.

Example

If a rigid body has

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

and experiences net torque

$$\tau=6,\mathrm{N,m},$$

then

$$\alpha=\frac{\tau}{I}=3,\mathrm{rad/s^2}.$$

Mass distribution changes the response

For the same torque, increasing the moment of inertia decreases the angular acceleration.

Moving mass farther from the rotation axis therefore makes a body harder to angularly accelerate even when its total mass is unchanged.

The equation $\sum\tau=I\alpha$ is specifically the simple fixed-axis form of rotational dynamics; more general rigid-body motion requires a fuller angular-momentum treatment.