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

Contact Forces

We've now seen four different types of forces: weight, normal, tension, and static friction. Weight is different from the others in two different ways.

First, weight is a noncontact force: that is, you don't have to be in contact with the Earth in order to feel the force of gravity. The moon feels the force of Earth's gravity even though it is many miles away and separated by vacuum. Astronauts in orbit also feel the force of Earth's gravity. (No, they're not actually weightless, as we'll see in Weightlessness.)

Gravity is the only noncontact force we'll look at in this semester; all other forces we'll see are contact forces. An object can only exert a contact force on another object if the two objects are touching each other. This makes identifying the forces on an object a little easier: we start by listing all the objects that are touching that object, and go from there.

For example, when you throw a ball, you exert a normal force on the ball until the moment it leaves your hand. Once it’s in the air, however, you no longer exert any force on the ball because you are no longer touching it. (This is a force, after all, not The Force™).