Although the first thought that crosses your mind is probably “ouch, that hurt” rather than “this is a great example of Newton’s third law,” both statements are true. If you have ever stubbed your toe, you have noticed that although your toe initiates the impact, the surface that you stub it on exerts a force back on your toe. Ask students which forces are internal and which are external in each scenario.

Explain how forces can be classified as internal or external to the system of interest.

Introduce the concepts of systems and systems of interest.

Start a discussion about action and reaction by giving examples.