Friction

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Friction

• Friction Problem Situations

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Friction

• Friction Ff is a force that resists motion
• Friction involves objects in contact with each
  other.
• Friction must be overcome before motion occurs.
• Friction is caused by the uneven surfaces of the
  touching objects. As surfaces are pressed
  together, they tend to interlock and offer
  resistance to being moved over each other.

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Friction

• Frictional forces are always in the direction
  that is opposite to the direction of motion or to
  the net force that produces the motion.
• Friction acts parallel to the surfaces in
  contact.

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Types of Friction

• Static friction maximum frictional force
  between stationary objects.
• Until some maximum value is reached and motion
  occurs, the frictional force is whatever force is
  necessary to prevent motion.
• Sliding or kinetic friction frictional force
  between objects that are sliding with respect to
  one another.
• Once enough force has been applied to the object
  to overcome static friction and get the object to
  move, the friction changes to sliding (or
  kinetic) friction.
• Sliding (kinetic) friction is less than static
  friction.

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Types of Friction

• Static and sliding friction are dependent on
• The nature of the surfaces in contact. Rough
  surfaces tend to produce more friction.
• The normal force (Fn) pressing the surfaces
  together the greater Fn is, the more friction
  there is.

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Types of Friction
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Types of Friction

• Rolling friction involves one object rolling
  over a surface or another object.
• Fluid friction involves the movement of a fluid
  over an object (air resistance or drag in water)
  or the addition of a lubricant (oil, grease,
  etc.) to change sliding or rolling friction to
  fluid friction.

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Coefficient of Friction

• Coefficient of friction (?) ratio of the
  frictional force to the normal force pressing the
  surfaces together. ? has no units.
• Static
• Sliding (kinetic)

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Coefficient of Friction
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Horizontal Surface Constant Speed

• Constant speed a O m/s2.
• The normal force pressing the surfaces together
  is the weight Fn Fw

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Horizontal Surface a gt O m/s2
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Horizontal Surface a gt O m/s2

• If solving for
• Fx
• Ff
• a

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Horizontal Surface Skidding to a Stop or
Slowing Down (a lt O m/s2)

• The frictional force is responsible for the
  negative acceleration.
• Generally, there is no Fx.

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Horizontal Surface Skidding to a Stop or
Slowing Down (a lt O m/s2)

• Most common use involves finding acceleration
  with a velocity equation and finding mk
• Acceleration will be negative because the speed
  is decreasing.

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Horizontal Surface Skidding to a Stop or
Slowing Down (a lt O m/s2)

• The negative sign for acceleration a is dropped
  because mk is a ratio of forces that does not
  depend on direction.
• Maximum stopping distance occurs when the tire is
  rotating. When this happens, a -msg.
• Otherwise, use a -mkg to find the
  acceleration, then use a velocity equation to
  find distance, time, or speed.

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Down an Inclined Plane
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Down an Inclined Plane

• Resolve Fw into Fx and Fy.
• The angle of the incline is always equal to the
  angle between Fw and Fy.
• Fw is always the hypotenuse of the right triangle
  formed by Fw, Fx, and Fy.

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Down an Inclined Plane

• The force pressing the surfaces together is NOT
  Fw, but Fy Fn Fy.
• or

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Down an Inclined Plane

• For constant speed (a 0 m/s2)

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Down an Inclined Plane

• To determine the angle of the incline
• If moving
• If at rest

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Up an Inclined Plane
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Up an Inclined Plane

• Resolve Fw into Fx and Fy.
• The angle of the incline is always equal to the
  angle between Fw and Fy.
• Fw is always the hypotenuse of the right triangle
  formed by Fw, Fx, and Fy.

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Up an Inclined Plane

• Fa is the force that must be applied in the
  direction of motion.
• Fa must overcome both friction and the
  x-component of the weight.
• The force pressing the surfaces together is Fy.

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Up an Inclined Plane

• For constant speed, a 0 m/s2.
• Fa Fx Ff
• For a gt 0 m/s2.
• Fa Fx Ff (ma)

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Pulling an Object on a Flat Surface
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Pulling an Object on a Flat Surface

• The pulling force F is resolved into Fx and Fy.

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Pulling an Object on a Flat Surface

• Fn is the force that the ground exerts upward on
  the mass. Fn equals the downward weight Fw minus
  the upward force Fy from the pulling force.
• For constant speed, a 0 m/s2.

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Simultaneous Pulling and Pushing an Object on a
Flat Surface
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Simultaneous Pulling and Pushing an Object on a
Flat Surface
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Pushing an Object on a Flat Surface
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Pushing an Object on a Flat Surface

• The pushing force F is resolved into Fx and Fy.

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Pushing an Object on a Flat Surface

• Fn is the force that the ground exerts upward on
  the mass. Fn equals the downward weight Fw plus
  the upward force Fy from the pushing force.
• For constant speed, a 0 m/s2.

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Pulling and Tension

• The acceleration a of both masses is the same.

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Pulling and Tension

• For each mass
• Isolate each mass and examine the forces acting
  on that mass.

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Pulling and Tension

• m1 mass
• T1 may not be a tension, but could be an applied
  force (Fa) that causes motion.

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Pulling and Tension

• m2 mass

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Pulling and Tension

• This problem can often be solved as a system of
  equations
• See the Solving Simultaneous Equations notes for
  instructions on how to solve this problem using a
  TI or Casio calculator.

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Revisiting Tension and Friction
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Revisiting Tension and Friction

• For the mass on the table, m1

• For the hanging mass, m2
• The acceleration a of both masses is the same.

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Revisiting Tension and Friction