To analyze forces clearly, isolate one object from its surroundings and draw only the external forces acting on that object. The result is a free-body diagram.

Isolate the object

Represent the selected object simply, often as a point or box. Everything else belongs to the surroundings.

Each external interaction with the surroundings becomes one force vector acting on the selected object.

Label forces by interaction

Useful labels identify the physical source of the force: weight, normal force, tension, friction, applied force, and so on.

A velocity or acceleration arrow is not a force and should not be included as one.

Choose axes

Coordinate axes can be chosen to simplify the forces. On an inclined plane, for example, one axis is often taken parallel to the surface and the other perpendicular to it.

Each force can then be resolved into components.

From diagram to equations

The net force is obtained by adding the force vectors:

$$\mathbf F_{\mathrm{net}}=\sum_i\mathbf F_i.$$

Component by component,

$$F_{\mathrm{net},x}=\sum_iF_{i,x},\qquad F_{\mathrm{net},y}=\sum_iF_{i,y}.$$

A good free-body diagram separates the physical question “what interactions act on this object?” from the later mathematical question “what motion follows from their vector sum?”