Question

What is the equilibrium rule?

Answer 1

There is no acceleration whatsoever.

Explanation:

When an object or a system is in equilibrium, it must not be accelerating whatsoever. That is, there is neither translational accretion nor angular acceleration. This is the same as saying:

1.) `vec F_"net" = Sigma vecF_i= 0`
The net force or sum of all forces acting on the system is zero.
Hence, there no change in velocity. The system can be either be at rest or moving at a constant speed in a straight line only.

2.)`vec tau_"net"= Sigma vectau_i= 0`
The net torque or sum of all torques acting on the system is zero.
Hence, there is no change in angular velocity. It can be zero rotation, or rotating at a constant angular speed in a fixed orientation only.