The net forces of acting on the body need to be zero. This is also applicable for the simply supported beams. This means that the support reaction is also counted in making the net force zero.

Equilibrium of a Rigid Body topics include: Rigid body equilibrium conditions, two and three force members, free body diagrams, constraints and statical determinacy. A rigid body is in equilibrium when the sum of all forces acting on it is zero. The body must also be in translational and rotational equilibrium.  Here are some other conditions for a rigid body to be in equilibrium: The body's linear and angular acceleration are zero relative to an inertial frame of reference. The body is not changing its linear or angular momentum. The body is not moving because its forces and torques are... Show more

Equilibrium of a Rigid Body topics include: Rigid body equilibrium conditions, two and three force members, free body diagrams, constraints and statical determinacy.

A rigid body is in equilibrium when the sum of all forces acting on it is zero. The body must also be in translational and rotational equilibrium. 

Here are some other conditions for a rigid body to be in equilibrium:
The body's linear and angular acceleration are zero relative to an inertial frame of reference.
The body is not changing its linear or angular momentum.
The body is not moving because its forces and torques are balanced. 

A resting object is also in equilibrium because it has no rotation, and the sum of the torques exerted on it is zero.

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