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This text is meant to accompany class discussions. It is not everything there is to know about uniform circular motion. It is meant as a  prep for class. More detailed notes and examples are given in the class notes, presentations, and demonstrations (click here.)

Click for the questions that go with this reading
3rd Law


Newton's 3rd law says:

For every action, there is an equal and opposite reaction.

Simplistically this means if you push on me I'll push back. But for physicists, it means forces come in pairs. There is an "action" force and a "reaction" force. When standing on the ground the pull of the Earth generates a downward force called weight. But you don't move downwards, This is because the Earth exerts an up upward, "reaction," force. These two forces touch each other. They are a force pair.

When you push horizontally on a chair hard enough for the chair to slide across the floor, you are applying an action force to the chair and the chair is applying a reaction force to you. These two forces are a force pair because they touch each other. The floor resists the chair's sliding through friction between it and the floor. However, the frictional force is not a force pair to your pushing. It is a force pair to the chair because it is touching the chair.

Some force pairs do not need to touch each other like the examples above. These force pairs are caused by what are called action at a distance forces. Magnetism is an example. However, we will generally not consider these action at a distance forces ...yet. For now look for forces that make surface contact with each other to make up the force pair refered to in Newton's 3rd law..

Example #1

Describe a force pair in the animation below.


Example #2

Example #3


Choose the best answer. It does not matter how many questions you miss. What matters is that you learn from any mistakes.





by Tony Wayne ...(If you are a teacher, please feel free to use these resources in your teaching.)