Friction

Summary

• Definition
  • Friction is a catchall word that refers to any force that resists relative tangential motion (or intended motion).
  • "Relative tangential motion" is a fancy way to say "slipping" or "sliding".
  • Its direction is opposite the relative velocity (or intended velocity).
• Types
  • Dry friction
    • The resistive force between solid surfaces in contact that resists their relative tangential motion.
    • "Friction" is often synonymous with "dry friction".
  • Viscous friction
    • The resistive force between surfaces in relative motion through a fluid (liquids & gases).
    • Air resistance or aerodynamic drag is a type of viscous friction.
    • Viscous friction is dealt with in a different section of this book.
  • 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.
    • Rolling resistance is dealt with in a different section of this book.
• 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)
      • is also known as starting friction
    • 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
      • is also known as sliding friction or dynamic friction
    • Equations

      f_s ≤ μ_sN

      Where

      fs =	static friction force
      μs =	coefficient of static friction

      f_k = μ_kN

      Where

      fk =	kinetic friction force
      μk =	coefficient of kinetic friction

• Dry friction is largely independent of…
  • speed once an object is moving.
  • the amount of area in contact.
  • surface roughness.
    • When a standardized exam uses a phrase like "rough surface" it almost certainly implies that friction is not negligible for a particular problem. This does not mean that roughness is a factor that affects dry friction.
    • Protrusions (asperities)…
      • may provide ledges where one surface can rest upon another and apply a normal force. Normal is not friction.
      • may result in abrasion, where one surface plows or digs into the other. Abrasion is not friction.
    • Ice, glass, and rubber can all be made smooth but ice has coefficients of friction that are low, glass medium, and rubber high. The material is what determines the amount of friction, not its surface texture.
    • Sanding a slippery surface may increase its friction by removing a lower friction coating and exposing a higher friction substrate. The opposite effect is also possible.