1. Newton’s 3rd law

2. But first, REVIEW Newton’s 1st Law:
An object at rest will stay at rest, an object in motion will stay in motion, unless acted upon by a net force
Newton’s 2nd Law:
An object’s acceleration is proportional to the force applied to it
More force, more acceleration
Less force, less acceleration
An object’s acceleration is inversely proportional to its mass
More mass = less acceleration
Less mass = more acceleration

3. Forces and Interactions
Newton’s 3rd Law of Motion:
Whenever one object exerts a force on a second object, the second object exerts an equal and opposite force on the first object
A force is a push or pull
Forces are not individual things but always occur in PAIRS
Action Force
Reaction Force
An action force cannot exist without a reaction force…and vice versa

4. Action – Reaction Pairs
Forces act in action-reaction pairs
Equal in magnitude
Opposite in direction
Example: A man pushes on a wall and the wall pushes on him back. Why doesn’t he move?
ACTION
ACTION
REACTION

5. What happens when a ball is dropped?
When a basketball is dropped, why does it bounce back up?
The ball exerts a force (weight) onto the ground
The ground exerts a force back on the ball

6. What happens when a ball is dropped?
If the action-reaction pairs are EQUAL and OPPOSITE, don’t they just cancel each other out?
If the forces cancel out, then the net force is equal to zero.
If the net force is zero, there is NO movement.
So why does the ball move??
The ball has less mass than the ground
Less mass = more acceleration