Newton's third law

Last time

Before we start: last time, in Moments, balance and stability, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

You cannot push without being pushed

Stand on a skateboard and push against a wall. You roll backwards. You pushed the wall, but the wall pushed you. Forces never come alone. They come in pairs.

Newton's third law

When object A exerts a force on object B, object B exerts an equal and opposite force on object A. The two forces are called a force pair, or action and reaction. They are equal in size, and they act in opposite directions. They are always the same type of force: both gravitational, both contact, or both electric.

The key rule

Here is the rule that students forget. The two forces in a pair act on different objects. So they can never cancel each other out: they are acting on two separate things. If two forces act on the same object, they are not a third law pair.

A book on a table

A book rests on a table. The Earth pulls the book down: that is its weight. The table pushes the book up. These two are equal and opposite, but they are not a pair, because both act on the book. The pair of the weight is the book pulling the Earth upwards.