Chapter 10, Section 4
Page 393

 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.

• Newton’s First 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

• If one object exerts a force on another object …
Ball
…then the second object exerts a force of equal strength in the opposite direction on the first object.
Head

• When the dog leaps, it pushes down on the ground (action force)
• The ground pushes the dog into the air
(reaction force)

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

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

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

• 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

• 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)

• 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?

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:

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

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