\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \tau{\uptau}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\) \(\def \ccw{\circlearrowleft}\) \(\def \cw{\circlearrowright}\)
Chapter 1: Equilibrium
2.

Our First Forces

There are many types of forces. Here are three to start:

Gravity

Weight
\(W\)
Newtons (N)

This apple weighs about 1 Newton

The force of gravity on an object is called its weight. On Earth, the weight always points downward. The SI unit of force is the Newton (abbreviated as N, and named after Isaac Newton); a medium-sized apple (appropriately enough) has a weight of about 1 Newton.

Normal Force

Normal Force
\(N\)
Newtons (N)
The normal force is simply a push by one object touching another. The word normal here doesn't mean "ordinary"; instead it's a fancy math word that means "perpendicular", because the normal force always points in a direction perpendicular to the surface of contact between the two objects. A push doesn't have to be an active thing: the floor pushes upward on you when you walk, for example.

Tension

A lamp hanging from a rope from the ceiling. An upward arrow says 'tension on lamp', while a downward arrow says 'tension on ceiling'

Tension
\(T\)
Newtons (N)
When a rope or something similar pulls on an object, the pull force is called tension. If the tension is provided by something thin like a rope, then the tension force will point along the rope. Ropes exert tension at both ends (so on the lamp and on the ceiling in the example here), and if the rope is very light compared to the objects it’s attached to (what physicists call a "massless rope"), the rope will exert the same tension at both ends.