A system is in dynamic equilibrium when all the forces and torques (about any axes) it experiences cancel each other, so that the centre of mass of the system is stationary and the system does not experience any rotation. Therefore two conditions are attached for the dynamic equilibrium of the system:
- translational equilibrium: no net force (in any direction) on the system.
- rotational equilibrium: no net torque (about any axis) on the system.
The second condition is also referred to as the principle of moments: the net torque about the axis of rotation of a body in rotational equilibrium is zero (i.e. the sum of the torques contributing to clockwise rotation is equal to the sum of the torques contributing to anticlockwise rotation).
