From our studies of the work of Professor Newton,
we have learned that
F = M a .
That is, the product of an object's mass and its acceleration
is equal to the net force acting on it.
An object in equilibrium is an object that has no acceleration.
In other words, it may be moving in a straight line at a speed
that does not change, or it may be just lying there before us.
In either case, since the object has no acceleration, we glance
at Newton's formula, and we instantly realize that the net force
on the object must be ZERO.
The object behaves just as if there were NO forces acting on it at all.