1. Newton’s Third Law of Motion
Chapter 10, Section 4
Page 393

2. Objectives for 10.4 State Newton’s third law of motion.
Be able to identify the action forces and reaction forces acting on an object.
Explain how an object’s momentum is calculated.
State the law of conservation of momentum.
Use the conservation of momentum to predict the velocity of an object after a collision.

3. Review Newton’s First Law? Newton’s Second Law?
(Inertia) An object at rest will stay at rest, or an object in motion will continue that motion unless acted upon by an outside force. (Inertia – resists a change in velocity)
Newton’s Second Law?
F = ma; acceleration of an object depends on the net force acting on the object and the object’s mass

4. Newton’s Third Law of Motion
If one object exerts a force on another object …
…then the second object exerts a force of equal strength in the opposite direction on the first object.
Ball
Head

5. Action-Reaction Pair
When the dog leaps, it pushes down on the ground (action force)
The ground pushes the dog into the air (reaction force)

6. Action-Reaction Pair
Action on the wall
When a skateboarder pushes against a wall (action force)
The wall pushes the skater in the opposite direction (reaction force)
motion
Reaction on the skater

7. Will there always be movement?
The mysterious floating Aardvark No
The Aardvark’s weight is the action force (normal force)
The normal force of Earth is the reaction force
Normal Force of Earth –
Pushing back on the Aardvark
Weight – Action Force
Pulling toward Earth

8. Will the opposite forces cancel?
No, the forces are on different objects
The action force is on the ball
The reaction force is on the wrists

9. Momentum Momentum is the “quantity of motion”
momentum = mass X velocity
It is the product of the object’s mass and velocity.
A vector quantity:
Magnitude and direction

10. Momentum Momentum can also be referred to as “mass in motion”
The more the mass the more the momentum (an elephant vs. a mouse)
The more the velocity the more the momentum (running vs. walking)

11. Momentum vs. Inertia
Momentum is a measurable vector quantity (product of mass and velocity)
Inertia is dependent on mass, but it is an object’s resistance to a change in velocity
What has more momentum: a 200 pound man running at 1 mph or a 65 pound girl running at 4 mph? Why?
What has more inertia?

12. Calculating Momentum momentum = mass X velocity
Which has more momentum: a 3.0-kg sledgehammer swung at 1.5 m/s or 4.0-kg sledgehammer swung at 0.9 m/s?
Momentum of first hammer:
Momentum of second hammer:

13. Law of the Conservation of Momentum
The total momentum of any group of objects remains the same, or is conserved, unless outside forces (like friction) act on the objects
activity

14. Conservation of Momentum-
Conservation of Momentum