Force Equilibrium

Take a look at the figure below.

The crate, having weight W, is initially supported by two blocks (left). In the FBD of the crate (right) the blocks have been replaced with two reaction forces, R1 and R2

Force equilibrium is said to exist when there are no unbalanced forces acting on a body; it is a necessary condition for the overall static equilibrium of a body (i.e. the body does not move). This can be denoted by the following vector equation: For the case of the crate on two blocks this requires that:
R1 + R2 = W
It should be noted that having the sum of all forces on a body equal to zero is a necessary but not sufficient condition for static equilibrium. Another condition must be met, which will be discussed later in the module.