Equilibrium of Rigid Bodies

It is tempting to think that a body lying perfectly still on a table experiences no force at all — but this is not quite right. Gravity certainly still pulls the body downward, and the table's own normal reaction force pushes upward on it with exactly equal magnitude; these two forces simply cancel each other exactly, so that no net force, rather than no force whatsoever, acts on the resting body. Precisely the same distinction applies in rotational terms, using torque (moment of force) rather than force: a body can have several individual torques acting on it that cancel to give no net torque, even though none of the individual torques is itself zero. …