\(\def \u#1{\,\mathrm{#1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \tau{\uptau}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\) \(\def \ccw{\circlearrowleft}\) \(\def \cw{\circlearrowright}\)
Chapter 1: Equilibrium
8.

Contact Forces

Which of these could be exerting a force on the apple?
  • the floor
  • the box
  • the person
Even though the person and the floor are both influencing the apple, only the box is touching it, and so only the box can be exerting a force on it (though it might not be).
A stick figure of a baseball pitcher throwing a ball. The ball has left the pitcher's hand, and a single arrow labelled
The pitcher is not exerting a force on the ball right now. The only force on the ball is weight.

All forces exist between two different objects, and with the exception of weight (and two other forces we will discuss later), those two objects must be in contact with one another for the force to exist. If you want to identify the forces acting on an object, then, you can start by figuring out what is touching the object.

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™).

A picture of an alien that looks suspiciously like Yoda from behind, holding out a hand. A rock in front of the alien is hovering above the ground.
A noncontact force that we won't be studying here.

Weight is a noncontact force, because you don't have to be in contact with the Earth in order to feel the force of gravity. Heck, 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.)