Challenging Assumptions about Motion Newtons First Law

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Section 8.3

• Challenging Assumptions about Motion Newtons
  First Law

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Summary Of Laws

• Laws were developed by Sir Isaac Newton.
• 1st Law An object at rest tends to stay at rest
  unless acted on by an outside force. An object
  in motion tends to stay in motion unless acted on
  by an outside force.

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Summary of Laws

• 2nd Law The acceleration of a body is directly
  proportional to the force applied to it and
  inversely proportional to its mass

4
Summary of Laws

• 3rd Law For every action, there is an equal and
  opposite reaction.

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Newtons First Law

• We need to understand Inertia- tendency of an
  object in motion to move or a stationary object
  not to move.
• Ex If a soccer ball is sitting on the ground,
  its going to stay. If I kick the ball, then it
  will keep moving.

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Newtons First Law

• What happens to the ball when I kick it into the
  air?
• It moves in a straight line in the air.
• Is my kick the only force acting on the ball?
• No. Gravity pulls it back to ground.

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Newtons First Law

• Air resistance also acts as a Friction -
  resistance to motion when masses rub each other.
• When you ride your bike, you feel the air
  resistance against your face.

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Newtons First Law

• So there is more than one force acting on the
  ball.
• What are they?
• Gravity and Friction.

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Friction

• The slicker the surface, the less friction.
• Thats why you cant stand up very well on an ice
  rink.
• Thats also why a ball doesnt roll very well on
  a shaggy carpet.

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Balanced Forces

• When 2 or more forces are equally strong and
  exert force in opposite directions.
• When forces are balanced, then an object is at
  rest.
• When forces are balanced, then the object is in
  equilibrium.

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Unbalanced forces

• Forces that are not being balanced by forces of
  equal strength and in opposite direction.
• Always cause a change in motion.

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Questions- Copy and Answer

• State each of Newtons three laws of motion in
  your own words.
• Define inertia.
• Explain the differences between balanced and
  unbalanced forces. Give two examples of each.