Summary

• Definition
  • Friction is the force between surfaces in contact that resists their relative tangential motion.
  • "Relative tangential motion" is a fancy way to say "slipping".
  • Its direction is opposite the relative velocity (or intended velocity).
• Types
  • Dry Friction
    • The resistive force between clean dry solid surfaces.
    • The phenomena one normally associates with the word friction. Friction is normally synonymous with dry friction.
  • Viscous Friction
    • The resistive force between surfaces in relative motion through a fluid (liquids & gases).
  • Rolling Resistance
    • The resistive force experienced by rolling objects.
    • Since rolling does not does not necessarily involve slipping, rolling resistance is not really a form of friction.
• Factors affecting dry friction
  • Dry friction is directly proportional to the normal force between the two surfaces in contact.
  • Dry friction depends on the materials in contact. This factor is measured by the quantity known as the coefficient of friction which is …
    • the ratio of the friction force to the normal force.
    • unitless
    • always greater than 0
    • usually less than 1 for most everyday materials
  • Dry friction is subdivided into two types.
    • Static friction …
      • occurs when the two surfaces in contact are not in relative motion; that is, when one surface is stationary relative to the other surface,
      • varies in strength from zero (when no external force is trying to force slippage) to some maximum value (just before slippage occurs)
    • Kinetic friction …
      • occurs when two surfaces in contact are in relative motion; that is when one surface is slipping or sliding across another surface,
      • is always weaker than the maximum static friction.
• Factors that don't affect dry friction
  • Friction is largely independent of surface roughness (despite what you may have read in other textbooks).
    • Protrusions or rough spots may provide microscopic ledges where one surface can rest upon another and apply a normal force. This is not friction.
    • The friction associated with sandpaper is no greater than the friction associated with quartz. Friction and abrasion are different phenomena.
    • Ice, glass, and rubber can all be made smooth but ice has a low coefficient of friction, glass a medium coefficient, and rubber a high coefficient. The material is what determines the amount of friction, not is surface texture.
    • Sanding a slippery surface may increase its friction by removing the low friction surface material and exposing an underlying high friction material.
  • Friction is independent of speed once an object is moving.
    • Faster does not mean more friction.