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Background Information
Newton’s Law of Universal Gravitation
The Principia - Mathematical
Principles of Natural Philosophy
 Newton’s three laws of motion is the
foundation for his Universal Law of
Gravitation

Newton’s Laws Quick Summary
1st Law:
 An object at rest stays at rest
 An object in motion stays in motion
with the same velocity, if no net force
 2nd Law:
 Accelerations are do to the net forces
on an object (mass)
 3rd Law:
 Action and reaction pair of forces

Newton’s 3rd Law (recap)
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Law III: To every action there is always
an equal and opposite reaction: or the
forces of two bodies on each other are
always equal and are directed in
opposite directions.
When one body exerts a force on a
second body, the second body exerts an
equal and opposite force on the first.
TWO objects and TWO forces
Forces always come in pairs
Newton’s 3rd Law
TWO Forces and TWO objects
(TWO different F.B.D.)
 IF object A pushes/pulls on object B
THEN object B pushes/pulls on object A in
the opposite direction
 Object A push/pull – Action Force
 Object B push/pull – Reaction Force

Newton’s 3rd Law Examples
The action force will be stated
 Identify the reaction force
 Draw the reaction force vector

Newton’s 3rd Law Examples
 Flower
hand
pulls on
Newton’s 3rd Law Examples
 Ball
bat
hits the
Newton’s 3rd Law Examples
The Earth pulls on the skydiver.
The skydiver pulls on the Earth.
Newton’s 3rd Law Examples
Draw a F.B.D. of both objects and
identify the action force and reaction
force.
 Mr. Z. kicks a ball across to floor.
What is the action force and reaction
force?
 Action Force: Mr. Z. kicking the ball.
 Reaction Force: The ball pushing on
Mr. Z.

Free Body Diagrams

Mr. Z.
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Tennis Ball
Newton’s 3rd Law Examples
One last example …
 This one is simple but a bit tricky!

Newton’s 3rd Law
You (a student) sitting in a chair.
 Identify all action-reaction pairs
 Draw a free body diagram from each
object
 Question: How many objects are
involved in this situation?
 Three!
 You (the student), chair and the Earth
 Labeling the action-reaction pairs on
the F.B.D.

Homework
Newton’s 3rd Law Practice
 Due Tues. 11/25/14
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Newton’s 3rd Law Examples
You (a student) sitting in a chair.
 What are the action forces (two)?
 1) The Earth pulling you down
 2) You pushing on the chair
 These are both ACTION forces!
 What are the reaction forces?
 1) You pulling on the Earth
 2) The chair pushing on you

Free Body Diagrams
Homework
Newton’s 3rd Law Practice
 Due Tues. 11/25/14
