Mechanical problems repeatedly use a small set of common force models. Each force should be introduced by the physical interaction that produces it, not merely by matching a memorized formula.

Weight

Near Earth's surface, gravity exerts the force

$$\mathbf W=m\mathbf g,$$

where $\mathbf g$ points downward. Mass and weight are different quantities: mass measures inertia, while weight is a force.

Normal force

When two surfaces press against each other, the normal force acts perpendicular to their contact surface.

Its magnitude is whatever the contact constraint requires; it is not automatically equal to the object's weight.

Tension

An ideal taut rope or cable pulls along its own direction. The force it transmits is called tension.

In an ideal massless rope passing over frictionless supports, the tension magnitude is the same along the rope.

Friction

When contacting surfaces resist relative tangential motion, they exert friction parallel to the contact surface. Its direction opposes relative sliding or the tendency to slide at the contact.

The magnitude of friction is not determined merely by naming the force. Static and sliding contacts obey different approximate models, and static friction in particular adjusts to the value required by the motion constraint up to a limiting value.

These force models are approximations with conditions. A free-body diagram should first identify the actual interactions and their directions; equations for their magnitudes come second.