Download Newton`s Third Law of Motion – Action and Reaction

Newton’s Third Law of Motion – Action and Reaction
Conceptual Physics
Objective:
The student will be able to:
• Define force as part of an interaction
• State Newton’s third law of motion
• Given an action force, identify the reaction force
• Explain why accelerations caused by action forces and reaction forces do
not have to be equal
6.1 Forces and Interactions
Newton realized that a force is not a thing in itself but part of a mutual action, an
between one thing and another.
𝐹𝑁
W = mg
In these interaction, forces always occur in
. The first force is called
the
force and the second is called the
force.
𝐹𝑁 is the Normal Force or Newton’s Third Law Force. So the block pushes on the table
and the table pushes on the block with a force equal and opposite resulting in a net
force of zero.
Action Force:
Reaction Force:
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6.2 Newton’s Third Law:
Newton’s third law states: Whenever one object exerts a force on a second object, the
second object exerts an equal and opposite force on the first object.
Newton’s third law is sometimes stated as:
For every action, there is an equal and opposite reaction.
Walking: You
with the floor when you walk on it. You
push on the floor, and the floor simultaneously pushes against you.
Talk with a partner: What happens when trying to walk on ice?
6.3 Identifying Action and Reaction
Action: Object A exerts a force on object B.
Reaction: Object B exerts a force on object A
Action: Tires Push Road
Reaction:
Action: Rocket Pushes Gas Reaction:
Action: Earth Pulls Ball
Reaction:
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6.4 Action and Reaction on Different Masses
If the action and reaction force are equal and opposite, why don’t both objects
accelerate at the same amount?
Example: The force the cannon exerts on the cannonball is exactly equal and opposite
to the force the cannonball exerts on the cannon.
Newton’s
2nd law of
motion.
Cannon: 𝑎 =
𝐹
𝑚𝐶
Cannonball: 𝑎 =
𝐹
𝑚𝐶𝐵
Answer: Suppose the force on the 25 kg cannonball by the gunpowder was 5000N.
(a) What is the acceleration of the cannonball?
(b) What is the force exerted by the cannonball on the cannon?
(c) What is acceleration of the 1250kg cannon?
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6.5 Do Action and Reaction Forces Cancel?
Talk with a partner: If the action and reaction forces are equal and opposite, shouldn’t
they cancel to zero?
?
6.6 The Horse-Cart Problem
Identify all the forces acting on the horse, the cart, and the ground? Does the cart
accelerate?
6.7 Action Equals Reaction
Applies ONLY to
.
How hard did the wall hit your hand? It hit just as hard as you hit the wall!
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