SECTION 5.2 FRICTION

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SECTION 5.2 FRICTION

•  Objectives
• Define the friction force.
• Distinguish between static and kinetic friction.

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INTRO

• Friction force opposing relative motion of 2
  objects that are in contact. Examples tires on
  road, shoes on ground. The direction of the
  force is parallel to the surface and in a
  direction that opposes the slipping of the 2
  surfaces.

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STATIC AND KINETIC FRICTION

• Kinetic Friction the force exerted on one
  surface by a second surface when the 2 surfaces
  rub against one another because one or both of
  the surfaces are moving. Also called Sliding
  Friction.
• From Old Book
• Sliding Friction force between 2 surfaces in
  relative motion. The force of sliding friction
  is less than that of static friction. Thus a car
  will stop faster if the wheels are not skidding.
• To keep a book sliding at constant velocity on a
  desk you must exert a constant force that is just
  the same size as the frictional force but in the
  opposite direction.
•  
• Experimentally it has been found that the force
  of friction depends primarily on the force
  pushing the surfaces together, FN, and on the
  nature of the surfaces in contact. FN is the
  force pushing the surfaces together (Normal or
  Perpendicular Force)

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STATIC AND KINETIC FRICTION

• Static Friction the force exerted on one
  surface by a second surface when there is no
  motion between the 2 surfaces.
•  
• From Old Book 
• Static Friction force that opposes start of
  motion between 2 surfaces. Static friction
  forces have maximum values.
• When the magnitude of your push on the box is
  greater than the max value of the static friction
  between the floor and the box, the box starts
  moving. When the box starts to move the force of
  friction decreases.

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STATIC AND KINETIC FRICTION

• Once your force is greater than this maximum
  STATIC Friction, the couch begins moving and
  KINETIC Friction begins to act instead of static
  friction.
•  
• You have Static Friction acting when trying to
  get an object moving and once it is moving you
  have Kinetic or Sliding Friction acting.
• Go Over example from book dealing with figure 5.8
  p. 126.

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A MODEL FOR FRICTION FORCES

• On what does a Frictional Force depend?
• The materials that the surfaces are made of play
  a role.
• The normal force between the 2 objects. The
  harder an object is pushed against the other, the
  greater the force of Friction.
•  
• If you pull a block along a surface at a constant
  velocity, according to Newtons Laws, the
  Frictional Force must be equal and opposite to
  the force with which you pull.
•  

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A MODEL FOR FRICTION FORCES

• Coefficient of Kinetic Friction - ratio of
  frictional force and the normal force between 2
  surfaces. It is indicated by the symbol ?. It
  is a constant that depends on the 2 surfaces in
  contact. This is for an object that is already
  moving.
•  
• Kinetic Friction Force the force exerted on one
  surface by a second surface when the 2 surfaces
  rub against one another because one or both of
  the surfaces are moving. It is equal to the
  product of the coefficient of kinetic friction
  and the normal force.
• FFKinetic µKFN FF µFN

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A MODEL FOR FRICTION FORCES

• Coefficient of Static Friction - ratio of
  frictional force and the normal force between 2
  surfaces. It is indicated by the symbol ?. It
  is a constant that depends on the 2 surfaces in
  contact. This is for an object you are trying to
  get to move.
• Static Friction Force - the force exerted on one
  surface by a second surface when there is no
  motion between the 2 surfaces. It is less than
  or equal to the product of the coefficient of
  static friction and the normal force.
• FFStatic µSFN FF µFN
• The static Friction Force will increase up to a
  maximum value before it is overcome and motion
  starts.

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A MODEL FOR FRICTION FORCES

• Do Example Problem 3 p. 128
• Fg mg 25(9.8) 245 N and Fg FN
• FPush µFN .2(245) 49 N (note .2 from
  table 5.1)
•  
• Do Practice Problems p. 128 17-21
• Show and Go Over Table 5.1 p. 129

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A MODEL FOR FRICTION FORCES
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A MODEL FOR FRICTION FORCES

• Do Example Problem 4 p. 129
• FPush doubles to 98 N and FF original FPush
  
• FNet FPush FF
• ma 98 49
• 25a 49
• a 1.96 m/s2
•  
• Do Practice Problems p. 130 22-26

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IMPORTANT THINGS ABOUT FRICTION

• Important Things to remember about Friction
• Friction acts in the direction opposite to the
  motion.
• The magnitude of the force of friction depends on
  the magnitude of the normal force between the 2
  surfaces.
• Multiplying the coefficient of static friction
  and the normal force gives you the maximum static
  friction force.
• Do 5.2 Section Review p. 130 28-32 (SKIP 27
  and 31)