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.

ReviewReview

• 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

• When a skateboarder pushes against a wall (action force)

• The wall pushes the skater in the opposite direction (reaction force)

motion

Action on the wall

Reaction on the skater

Will there always be movement?

• No

• The Aardvark’s weight is the action force (normal force)

• The normal force of Earth is the reaction force

Weight – Action Force

Pulling toward Earth

Normal Force of Earth –

Pushing back on the Aardvark

The mysterious floating Aardvark

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

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Conservation of Momentum