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Newton's Third Law

Newton's third law is about forces acting in pairs, we call one of these forces the action and the other the reaction.

If you hit a wall with your fist then you will feel a force on your fist as well as there being a force on the wall. When a bullet leaves a gun there is a force on the bullet but there is also a force on the gun and this makes it recoil.

In both the two examples, the two forces are equal and opposite although their effects on the objects may well be very different. In the example of a fist hitting the wall the effect on your fist is very different from the effect of your fist on the wall.

Even a person sitting on a stool is subject to this law, the action of them on the stool (their weight) is just balanced by the upward thrust of the stool on them. Think of the result if these two forces were not equal.

You can also use the example of the apple on a stalk (Figure 1) (see Newton's First Law). The force on the apple due to the stalk is the same as the force on the apple due to gravity and so the apple stays where it is.

Newton's third law can be expressed as:

Action and reaction are equal and opposite

or as:

When a force acts on one body an equal and opposite force acts on another body.

Notice that the two forces, the action and reaction, act on different bodies. If they acted on the same body it would be impossible to move anything.

Another demonstration of the law can be shown with the spring-loaded trolleys (Figure 2). If they are placed back to back and then the spring in one released they will both move off because there is a force acting on them both.

© Keith Gibbs 2020