Newton’s Third Law

Newton’s Third Law is all about situations where two objects are interacting with each other. A common way of stating Newton’s Third Law is to say that for every action there is an equal and opposite reaction.

This means different things in different situations.

If the two objects are stationary, such as a person sitting on a chair, the person’s weight is a downward force that acts on the chair and this is balanced by an equal and opposite upwards force generated by the chair (the normal contact force) to resist the person’s weight. The force generated by the chair is present only when the person is sitting on the chair.

If we think about the chair alone, an important feature of this situation is the chair’s inability to move (downwards) because it too is being resisted – by the ground underneath its legs (another normal contact force) And because the force applied and the resistance from the ground exactly balance each other, Newton’s First Law tells us that the chair will not change its state of motion.

Things are different when one of the two objects is free to move, such as a footballer kicking a football. At the moment of impact, we all know that the footballer applies a force to the ball. We probably also know that the ball applies a force onto the footballer’s foot (because if you kick a heavy ball, bare-footed, it can hurt). But what may be less obvious is that the force of the ball on the foot is exactly equal to the force of the foot on the ball. The only reason why the ball shoots off into the distance is because it is free to move away, NOT because it feels a greater force!

The footballer’s leg does not move in the opposite direction because the person’s anatomy (muscles and bones) and overall movement keep the leg moving forwards even though it is subjected to a backwards force. (Of course, if the ball had been made of concrete and fixed to the ground then it would have been a different story!)

The speed with which the football moves away will depend on the ball’s acceleration, and Newton’s Second Law tells us that the acceleration will be equal to the applied force divided by the mass of the ball.

That is as much as is needed for the AQA Trilogy course but for full GCSE Physics courses we need to recognise that Newton’s Third Law builds on both Newton’s Second Law (in dynamic situations) and Newton’s First Law (in static situations).

To get even more from Newton’s Third Law we need to think about a quantity called impulse, which is calculated by multiplying the applied force by the time for which it acts. This will give us the change in momentum for the object – and that is what we need to calculate the object’s speed after it has been acted on by a force.

Without using any mathematics, you might be able to appreciate the physics being described here if you think about firing a gun. When the gun is fired there is recoil, which pushes back onto the person who fired the gun. Exactly the same push is also applied to the bullet and, because the bullet has a much smaller mass, the bullet will move forwards much faster than the person moves backwards.

We will learn more about momentum in a future article.