In other words, it means that the resultant of all forces acting over the body, Res, in other terms, the sum of all forces applied to it, ΣF, is equal to zero. So statics is just a particular case of dynamics (ΣF = m.a), in which acceleration is zero.
If we want to put this equilibrium condition down in equations, we have to remmember that this would be a vectorial relation. So if the forces acting on the body have the same position, we will only need one equation (ΣF = 0).
Although, if the forces applied on the body have different directions, but at the same time on a single plane, we will need two equations:
ΣFx = 0
ΣFy = 0
Of course, we should never forget to set the reference system, (X and Y axis), the same as we do in dynamics.
If forces act in diverse directions, not contained in a single plane, we will need 3 equations: one for each of the directions in our tridimensional refrence system (x, y, z).
Given that we're using particles as models, it's kind of obvious that all forces acting on a body, they concur. In the next chapter (rigid body) that is what is going to change.Lets see an example: