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### 401-B: Newton's Third Law 1

• Topic Cluster: Dynamics
• Topic: Identifying Forces (Newton's Third Law)
• Objective: State Newton's Third Law and recognize examples of how it applies.
• Content: Newton's Third Law states that if A exerts a force on B, B exerts a force on A with equal magnitude and opposite direction.
• Level: 1

#### BACK to Ladder Identifying Forces (Newton's Third Law)

##### Three ways to write Newton's Third Law:
The Poetic Version
For every action there is an equal and opposite reaction.
Beautiful, famous quote.
Makes you sound smart.
It isn't clear what this actually means.
The Simple Version
If you push something, it pushes you back with the same force, called the reaction force.
Intuitive; captures the essence of Newton's Third Law in simple, easy language.
Too simple to use for complex physical models.
The Precise Version
Whenever A exerts a force on B, also exerts a force on A with equal magnitude and opposite direction.
These two forces are called an action-reaction pair.
Truly precise and correct, can be used for advanced physics models.
Theoretical and non-intuitive.

In this class, we will start by using the simple version in level 1, but quickly move to the precise version in level 2 and above. The poetic version you should memorize to sound smart, but it isn't very useful for solving physics problems.

1. Mr. Kuncik punches the wall with a force of 200 N. What force does the wall exert on Mr. Kuncik? What will Mr. Kuncik feel?
2. If Mr. Kuncik punched the wall to the right, what is the direction of the reaction force?
3. A massive linebacker slams Mr. Kuncik in the face with a force of 800 N. What force does Mr. Kuncik's face exert on the linebackers fist?
4. A truck is speeding on the highway, when it hits an innocent bug flying. SPLAT! During the collision, which exerted more force:
1. the bug
2. the windshield
3. they exerted the same force
Explain how you know:
5. Make up your own situation involving Newton's Third Law.