\(\def \u#1{\,\mathrm{#1}}\) \(\def \us#1{\,\mathrm{\scriptsize #1}}\) \(\def \abs#1{\left|#1\right|}\) \(\def \ast{*}\) \(\def \deg{^{\circ}}\) \(\def \tau{\uptau}\) \(\def \ten#1{\times 10^{#1}}\) \(\def \redcancel#1{{\color{red}\cancel{#1}}}\) \(\def \BLUE#1{{\color{blue} #1}}\) \(\def \RED#1{{\color{red} #1}}\) \(\def \PURPLE#1{{\color{purple} #1}}\) \(\def \th#1,#2{#1,\!#2}\) \(\def \lshift#1#2{\underset{\Leftarrow\atop{#2}}#1}}\) \(\def \rshift#1#2{\underset{\Rightarrow\atop{#2}}#1}}\) \(\def \dotspot{{\color{lightgray}{\circ}}}\) \(\def \ccw{\circlearrowleft}\) \(\def \cw{\circlearrowright}\)
Chapter 2: Laws of Motion
6.

Newton's Third Law

When you push on a table, it pushes back on you. When you pull on a rope, it pulls on you. This is the essence of Newton's Third Law:

If A exerts a force on B,
then B also exerts a force on A.
These forces have the same strength,
are the same type,
and point in opposite directions.

We call these two forces a force pair or force twins. If you know one force, you can find its twin by describing the force in a sentence, reversing subject and object, and reversing the direction. For example:

"The table pushes upward on the book."
"The book pushes downward on the table."

Or

"The Earth pulls down on me via gravity."
"I pull up on the Earth via gravity."

This can be counterintuitive.

Example

A truck and a mosquito collide on a highway. Which one feels the greater force?

ANSWER:According to Newton's Third Law, the mosquito exerts the same strength of force on the truck as the truck exerts on the mosquito! (Of course the effect of that force is very different in both cases, and we'll talk about that later.)