Newtons Third Law

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Newtons Third Law

• Physics 1010
• Dr. Don Franceschetti
• September 13, 2005

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Guide Questions

• What is Newtons third law of motion and how can
  I make use of it.
• What force do I exert on the Earth? Why do I not
  see an effect?
• What should I do if I cant avoid a car crash?

3
Review

• Q4.1 Is acceleration proportional to net force
  or does acceleration equal net force?
• Accelerationchange in velocity / time
• Fma
• Force is proportional to acceleration
• And they are measured in different units.

4
Review

• Q4.16 What is the weight of a 1-kilogram brick?
• E4.1 What is the net force on a Mercedes
  convertible traveling along a straight road at a
  steady speed of 100 km/h?
• E4.6 Is it possible to go around a curve in the
  absence of a force?

5
Review

• E4.8 When you throw a ball upward, which is
  larger, the weight of the ball or your upward
  force on the ball?
• E4.29 When you jump vertically off the ground,
  what is your acceleration when you reach your
  highest point?

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Review

• 4.P1 What is the greatest acceleration a runner
  can muster if the friction between her shoes and
  the pavement is 90 her weight?
• 4.P3 What is the acceleration of a 20-kg pail of
  cement that is pulled upward with a force of 300
  N?

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Newtons Third Law of Motion

• Whenever one object exerts a force on a second
  object, the second object exerts an equal and
  opposite force on the first.
• Forces result from the interaction of objects.
  One interaction implies two forces always acting
  on two different objects.
• It doesnt matter whether the objects are in
  motion or not!

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Action/Reaction Pairs

• You and the Earth.
• Shoe sole and floor.
• Car tire and road.
• Rocket and exhaust.
• Two cars crashing.
• Notice that the forces always apply to different
  objects.

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Mass differences often hide one force

• Fma, mF/a, aF/m
• So you dont notice the acceleration of the Earth
  or the backward motion of the road.
• But if you move about in a small boat
• Rifle recoil

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Newtons Three Laws

• Inertia, Fma, action/reaction
• Need to be used together
• Allow us to determine magnitude of a force if the
  other forces are known.

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Just a little bit about vectors.

• Parallelogram law.
• Vector quantities are those that add like
  displacements.

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Review

• Q5.9 The Earth pulls down on you with a
  gravitational force that you call your weight.
  Do you pull up on the Earth with the same amount
  of force?
• Q5.11 Identify the force that propels a rocket.

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For further study

• E5.3 You hold an apple over your head. (a)
  Identify all the forces acting on the apple. (b)
  When you drop the apple, identify all forces
  acting on it as it falls and the corresponding
  reaction forces. Neglect air drag.

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Review

• E5.13 Why does a rope climber pull downward on
  the rope to move upward?
• E5.23 Consider a tug of war on a smooth floor
  between boys wearing socks and girls wearing
  rubber soled shoes. Why do the girls win?