Friction and Tension

Weight and the normal force are on almost every object. Three more forces show up whenever something slides, hangs from a rope or is pushed.

Friction $f$: a rough surface resists an object sliding on it. Friction points along the surface, against the motion.

Tension $T$: a rope or a cable pulls along its own length, away from the object it is tied to.

Applied force $F$: a push or a pull from a hand, a motor or another object.

A crate pulled by a rope across a rough floor: four forces, each drawn from the crate.

A free-body diagram draws one object with every force acting on it, each arrow starting at the object. To find them, list what touches the object (the floor, a rope, a hand) and add its weight. In this course you draw the forces before you write any equation.