Download 10.4 Newton`s Third Law of Motion and Momentum

Newton’s Third Law of
Motion
Chapter 10, Section 4
Page 393
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.
Review
• 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
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
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)
Action-Reaction Pair
Action on
the wall
motion
Reaction on
the skater
• When a skateboarder
pushes against a wall
(action force)
• The wall pushes the
skater in the opposite
direction (reaction
force)
Will there always be movement?
• No
Normal Force of Earth –
Pushing back on the Aardvark
The mysterious floating
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
• 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
• 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 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?
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:
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
-
Conservation of Momentum