Static Equilibrium

Static equilibrium is the mechanical condition where both the net linear force and net torque on a system are equal to zero. Static equilibrium also requires the system to be at rest in its frame of reference. So, if the net torque and force equal zero but the object is moving(at a constant velocity), it's not technically in static equilibrium. This form of equilibrium is known as dynamic equilibrium.

If this confuses you, let's look at the image below

The net force on this pencil is 0. The normal force from the purple surface below the pencil is equal in magnitude to the weight of the pencil. The net torque is also 0 because the normal force exerts a torque at the pivot so the torque exerted by the normal force is 0. Similarly, the pencil's weight acts parallel to the pencil's radius so the torque exerted by the pencil's weight is also 0.

Therefore, the pencil is in static equilibrium.

Now, the pencil is no longer in static equilibrium. Why? Its net force is still 0 but now, its net torque isn't 0. The normal force still exerts no torque but its weight exerts a clockwise torque. If you observe this pencil, it will rotate back to static equilibrium, which in layman's terms means it will fall back over to where it was in the first image.

This phenomenon is known as unstable equilibrium. Unstable equilibrium occurs when a system is not in static equilibrium but its net torque and/or net force are such that it can return back to static equilibrium. In other words, unstable equilibrium occurs when a system isn't in static equilibrium but will go back to static equilibrium.