1. Chapter 2, Section 4 Newton’s Third Law
Forces
Chapter 2, Section 4
Newton’s Third Law

2. Newton’s Third Law of Motion
Newton’s Third Law of Motion – for every action there is an equal and opposite reaction.
Action-Reaction pairs
Jumping up in the air
Your feet are the action force, the ground pushing back is the reaction force
Rowing a boat in the water
Your pushing of the paddle in the water is the action force, the water pushing back on the paddle is the reaction force.

3. Do Action-Reaction Forces Cancel?
Detecting Motion
You cannot always detect motion.
Ex. – Gravity
Do Action-Reaction Forces Cancel?
As learned earlier: if two forces are balanced the forces cancel each other out.
Does not apply to Action-Reaction Forces because they are acting on different objects.
Ex. – volleyball player hitting the ball.
p. 57 in textbook

4. Momentum
Momentum = a quantity of motion found by taking Mass x Velocity. (see pg. 58 in book for example of the calculation)
Described by its direction and quantity (momentum).
The more the momentum, the harder it is to stop.
Mass also affects momentum.
Ex. You can catch a baseball going 20 m/s but you cannot stop a car going the same speed.

5. Conservation of Momentum
Conservation refers to the conditions before and after an event.
The law of conservation of momentum states that the momentum of an object is unchanged unless a force acts on it (like friction)
Only an outside force can change the momentum of an object.
Velocity may change after two objects collide, but the momentum remains the same.

6. Use textbook p. 61 to go through diagram with students.
Figure 18 in textbook, p. 60.