A body that is at rest, or is moving with constant velocity, is in a state of equilibrium.
Therefore the resultant force in any direction is also zero.
The converse of this statement is not necessarily true because, although forces with zero resultant cannot make and object move in a line they can cause an object to turn .
However a set of concurrent forces ( i.e. all passing through one point ) can never cause turning so, as at present we will deal only with concurrent forces, the problem of turning will not arise yet.
If the resultant is zero, the collected components in each direction must individually be zero i.e. X = 0 and Y = 0.
Applying this fact to a concurrent system in equilibrium, in which some forces are unknown, provides a method for finding the unknown quantities.
