Forces

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Forces

• Friction and Equilibrium

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Friction Force

• Static Frictionfriction force between an object
  at rest and the surface it is at rest upon
• Kinetic Frictionfriction force between an object
  in motion and the surface it is moving upon
• Which is greater?

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Friction

• The friction force depends upon the normal force
  and the coefficient of friction.

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Problem 1

• How much force is needed to keep a 1200 kg car
  moving at constant velocity on a level concrete
  road? Assume that the car is moving too slowly
  for air resistance to be important.

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Problem 2

• A force of 200 N is just sufficient to start a 50
  kg steel trunk moving across a wooden floor. Find
  the coefficient of static friction.

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Problem 3

• The coefficient of kinetic friction between a
  rubber tire and a wet concrete road is 0.5. A)
  Find the minimum time in which a car whose
  initial velocity is 50 km/h can come to a stop on
  such a road. B) What distance will the car cover
  in this time?

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Inclined Plane Problems

• An object rests on an inclined plane that makes
  an angle of ? with the horizontal.
• Label all the forces acting on the object.
• Rotate the x and y axes so that they align with
  the incline.
• Set up the equilibrium equations.

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Problem 1

• A woman whose shoes have leather soles and heels
  is able to stand without slipping on a wooden
  surface that makes an angle of 25 with the
  horizontal. What is the minimum coefficient of
  static friction for leather on wood?

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