FRICTION

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FRICTION

• Sliding and Static

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Sliding Friction

• Often called kinetic friction
• A force opposite to direction of motion
• Due to bumps in surfaces and electric forces

Surface under microscope
Ff
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Kinetic Friction is

• Dependent on nature of the two surfaces
• Directly proportional to the normal force between
  the surfaces
• Ff mkFN (not vector equation) (mu)
• Independent of area
• Independent of velocity

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Coefficient of friction mk

• Generally between zero and one
• Calculate from Ff / FN
• Can be more than one for special rubber
• Very low for ice, teflon, lubricated surfaces,
  ball bearings

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Coefficient of friction ms

• Coefficient of static friction
• Only applies at the instant of breakaway\
• ms Ff / FN

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Friction Good or Bad

• Mostly undesirable since reduces useful force and
  wastes energy
• Friction produces heat
• Necessary for walking!
• Necessary for braking

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Static Friction

• Force to start something moving
• Usually larger than kinetic friction for same
  surfaces
• Requires force to be exerted
• Before sliding begins, is equal and opposite to
  applied force
• msFN at breakaway only(otherwise less)

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Friction vs Applied Force
FA

• Starting with body at rest

msFN here only
Ff
Ff mkFN here
450
FA
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Kinetic Friction for Block

• Normal force equals weight on a level surface

FN
mg
Ff mkFN mk mg
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Which is Easier? Pulling or Pushing a Sled?
How do friction and normal force help us answer?
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What is the Friction Force?

• Block on an inclined plane

mkmgcosq
mg