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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.)

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.

 Question If every action has an equal and opposite reaction, then why does a baseball change speed so much when it is hit with a baseball bat? If the action and reaction forces were equal then wouldn't they cancel each other out -like when a student leans against a wall and neither changes velocity?   Understanding this comes down to understanding Newton's 2nd law, F=ma. The ball and the bat each experience the same force. Because the baseball is so much lighter than the bat, its acceleration, or change in velocity, is greater than the bats. A baseball's mass is 145 grams. A typcal baseball bat can have a mass up to 1000 grams. The bat's mass is nearly 7 times more than the baseballs. It's change in speed is 7 time less than the ball's. It is typical to say that an, "unblanced" force acts on the ball because the reactipon force's effects on the bat are not considered.   If a runner sprints in a race then we see the runner accelerate. The runner pushes on the Earth and the Earth pushes back with an equal and opposite force. We recognize is the runner's change in velocity. However, the Earth also changes velocity. Its size is so immense compared to the runner's, that we don't see or feel the velocity change. When a force is considered to be "unbalanced," it is just because the reaction force is not considered or observed.

Example #1
 Describe a force pair in the animation below. (Use a force pair that does not involve the Earth.)

 Think of the two forces that are touching each other. Describe a force pair in the animation below. (Use a force pair that does not involve the Earth.)

 When the car hits the rear of the truck, the truck's rear pushes on the car and the car pushes on the track. The force touch each other. Describe a force pair in the animation below. (Use a force pair that does not involve the Earth.)

Example #2
 A large truck breaks down out on the road and receives a push back into town by a small compact car as shown in the figure below. While the car is speeding up and pushing the truck, how does the force the car applies to the truck compare to the force the truck applies to the car?

 The 3rd law says all force pairs are equal and opposite. The force the car applies to the truck is EQUAL and opposite to the force the truck applies to the car. The 3rd law does no say anything special about whether or not the two bodies are changing speed. A large truck breaks down out on the road and receives a push back into town by a small compact car as shown in the figure below. While the car is speeding up and pushing the truck, how does the force the car applies to the truck compare to the force the truck applies to the car?

Example #3
 A large truck breaks down out on the road and receives a push into town by a small compact car as shown in the fugure below. The car and truck have reached a constant velocity. How does the force the car applies to the truck compare to the force the truck applies to the car?

 The 3rd law says all force pairs are equal and opposite. The force the car applies to the truck is EQUAL and opposite to the force the truck applies to the car. The 3rd law does no say anything special about whether or not the two bodies are traveling at a constant velocity. A large truck breaks down out o the road and receives a push into town by a small compact car as shown in the fugure below. The car and truck have reached a constant velocity. How does the force the car applies to the truck compare to teh force the truck applies to the car?