Forces in Equilibrium
At the beginning of this tutorial, it was mentioned that resultant forces cause objects to accelerate in a straight line. If an object is stationary or moving at constant velocity then either,
no forces are acting on the object, or
the forces acting on that object are exactly balanced.
In other words, for stationary objects or objects moving with constant velocity, the resultant force acting on the object is zero.
An object in equilibrium has both the sum of the forces acting on it equal to zero.