Newtons Third Law

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Newtons Third Law
2
Bell Work

• Explain why a competitive swimmer might wear a
  swim cap and shave their body hair.
• How would the acceleration of a bus be affected
  if more passengers were on the bus? Explain why.

3
Objectives

• Apply Newtons third law of motion.
• Identify the action force and reaction force
  between two objects.
• Distinguish between a pair of balanced forces and
  a pair of action and reaction forces.

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Newtons Third Law of Motion

• For every action by a force, there is an equal
  and opposite reaction by another force.
• If one object exerts a force on a second object,
  the second object exerts an equal and opposite
  force on the first object.

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Newtons Third Law of Motion

• There is an action force and a reaction force.
• These two forces are acting on different objects.

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Action Force exerted by The fish on the water
Reaction Force exerted by the water on the fish
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Action Force
Force applied by exhaust on the air launch pad.
Reaction Force
Equal Opposite force applied by the air
launch pad on the shuttle.
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Identify the action and reaction forces including
the objects on which they are exerted.
9
Identify the action and reaction forces including
the objects on which they are exerted.
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Examples of Newtons Third Law

• Give other examples of Newtons third law of
  motion.
• List the action and reaction force including the
  objects on which each force is exerted.

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Balanced Forces

• Equal and opposite forces only cancel each other
  when they are exerted on the same object.
• Action/reaction forces are exerted on two
  different objects!

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Balanced Forces

• When two people are pulling on opposite sides of
  a rope with equal force the forces are balanced
  and they cancel. Both forces are exerted on the
  same object, the rope.

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Action/reaction Forces

• As the people pull on the rope they stand on the
  ground. An action force is exerted on the ground
  by the persons feet. A reaction force is exerted
  by the ground on the persons feet.

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Action/Reaction Forces

• These two forces are exerted on different objects
  and do not cancel.

15
Identify other balanced and action/reaction
forces.

• In your notebook, write down two examples of
  balanced forces and two examples of
  action/reaction forces. Try to think of examples
  other than what we have discussed.

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Question

• Why does the space shuttle or a helicopter
  accelerate upward while the earth does not
  accelerate downward?
• What determines whether or not an object will
  accelerate?

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It is the reaction force and the water resistance
force that are balanced or unbalanced and
determine acceleration.
Action Force
Reaction Force
Water Resistance/Current
18
The action force exerted on the earth by the
helicopter on the earth is not enough to overcome
the earths inertia.
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Review

• Define and give an example of Newtons third law
  of motion.
• Identify the action and reaction forces when a
  blown up balloon that has not been tied is let go.

20
Review

• Explain the difference between action/reaction
  forces and balanced forces.