Newton's Third Law of Motion

Back to Physics Grade 9/10

Description

This is a basic physics tutorial that is targeted at GCSE (grade 9 and grade 10) standard exploring Newton's Third Law of Motion. The associated video tutorial is on its way.







Quick Recap

If you haven't learned about Newton's First and Second Laws of Motion, we suggest you do so first. So far we've learned that an object left to its own devices will continue to move at a constant velocity. If we apply to a force to it, there will be a resultant acceleration.

What is Newton's Third Law of Motion?

Newton's Third Law of Motion tells us what happens when one object exerts a force on another object. It states that for every action, there is an equal and opposite reaction. This means that:

When one object exerts a force on a second object,
the second object simultaneously exerts a force equal in magnitude
and opposite in direction on the first object.



Let's look at an example. A person swimming in a pool exerts a force on the water. This force moves the water down and backwards. At the exact same time, the water actually exerts a force that is the exact same size back onto the swimmer, but in the opposite direction - this pushes them forward and up i.e. is what allows them to swim through water. Mouse over the image below to see this in action.

The force that the water exerts on the swimmer is of the same magnitude and opposite direction as the force that the swimmer exerts on the water.




Action and Reaction Forces - what are they?

Sometimes the forces described above are referred to as action and reaction forces, whereby the one that's being exerted by the person is the action force, and the one that's being exerted by the water is the reaction force. Mouse over the image below to see this in action.

The force that the swimmer exerts on the water is the action force. The force that the water exerts on the swimmer is called the reaction force.



We learn more about Newton's Third Law of Motion when we discuss different types of forces. But for now, let's have a look at balanced and unbalanced forces.