1. Day 1 1-4-11 Unit 4 Topic: Intro to Newton’s 3rd Law
Objective: I will learn to describe force pairs within a system.
Science Starter:
Explain
how a rocket
lifts off the ground.

2. WE ARE USING CLASSROOM SET OF TESTS
Unit 4 Pretest
DO NOT WRITE ON TEST!!!
WE ARE USING CLASSROOM SET OF TESTS

3. Unit 4 Vocabulary
Newton’s 3rd Law of Motion : For every action force, there is a reaction force equal in strength and opposite in direction. Forces only come in pairs. (there can never be a single force!)
Force: A push or pull that can change motion.
Momentum: The mass of an object multiplied by its velocity.
Velocity: speed with direction. (a vector quantity)
Conservation of Momentum: Total momentum in a system cannot change. (exclude outside forces)

4. Who wins this contest?
Imagine a skateboard contest between Newton and an elephant. They can only push against each other, not against the ground. The fastest one wins. The elephant knows it is much stronger and pushes off Newton with a huge
force thinking it will surely win. But who does win?

5. ANSWER
Newton wins — and will always win. No matter how hard the elephant pushes, Newton always moves away at a greater speed. In fact, Newton doesn’t have to push at all and he still wins. Why?
You already know it takes force to make both Newton and the elephant move. Newton wins because forces always come in pairs. The elephant pushes against Newton and that action force pushes Newton away. The elephant’s force against Newton creates a reaction force against the elephant. Since the action and reaction forces are equal in strength and because of Newton’s second law of motion (a =F/m), Newton accelerates more because his mass is smaller.

6. Rules to follow when identifying action/reaction pairs:
1. Identify the objects that are systems of interest.
2. Draw each object separately. Place them in the correct position relative to other objects.
3. Identify every force. Draw the force vector on the object on which it acts. (Label each)
4. Identify the action/reaction pairs. A force goes with a force. Connect the two action/reaction pairs with a dotted line. When you’re done, there should be no unpaired forces.
5. Draw a free-body diagram for each object within the system. Include only the forces acting on the objects in your system.

7. Action / Reaction