Friction

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.

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