Forces Always Come In Pairs
Mr. Morin
A force is a push or a pull. There are only 4 types of forces in the universe:
 Gravitational Force
 Electromagnetic Force
 Strong Nuclear Force
 Weak Nuclear Force
It is not possible to have a single force! Forces always exist in pairs, between two different objects.
Simple rules about force pairs:
 The two forces are always equal in magnitude.
 The two forces are always pointed in opposite directions.
 The two forces always act on different objects.
 The two forces always start and end at the same time – they always have the same duration.
It is a good practice to write what is acting on what. For example, in a force diagram (or free body
diagram), instead of labeling the pull of a rope on a cart Ftension or just F, it should really be labeled
Ft By Rope On Cart (or Ft RC), to remind us that there is an opposite force, Ft By Cart On Rope (or Ft CR) acting on the
rope.
These force pairs are sometimes described as “action” and “reaction” forces; and Newton’s 3rd law is
somewhat misleadingly stated as: “Every action has an equal and opposite reaction.” These descriptions
are not precise physics descriptions, although they are used in almost all physics textbooks. These words
also have strong meanings outside of physics.
The following examples are NOT legitimate force pairs:
“I pushed Jimmy, so his brother pushed me”. Yes, there are two forces, but they violate all of the
above rules.
“I pushed Jimmy, so after school, he pushed me back, and it was the same force in the opposite
direction, SO THERE!” – Sorry, the two forces didn’t happen at the same time.
“Gravity pulls down on the book, and the table pushes back up on the book” – Surprisingly, this is
not a force interaction pair.
The gravitational force acts on the book by the earth, and the support force
acts on the book by the table. Each has its own equal and opposite force. The
earth pulls down on the book, and the book pulls up on
the Earth. The table pushes up on the book, and the
book pushes down on the table.
To describe the gravitational force between the Earth
and the Book, one would show Fg By Earth On Book, Fg EB and
Fg By Book On Earth, Fg BE. For the Book/Table pair, one would
use Fn By Table On Book, Fb TB and Fn By Book On Table, Fn BT.
Always remember Newton’s 3rd Law:
Forces always come in pairs.