Chapter XVIII: Appendix: I

REDUCING FRICTION

When two bodies are rubbed together the motion is opposed by a force called _friction_. When two surfaces slide against each other the friction between them is proportional to the force pressing them together. The _amount_ of friction depends upon the pressure of the bodies, the roughness of their surfaces and also slightly on their _adhesion_. The friction is the same regardless of the speed with which the surfaces slide over each other.

The _co-efficient of friction_ is the measure of friction and this is found by dividing the _force of friction_ by the _force pressing the surfaces together_. Here are a few co-efficients of sliding friction:

_Per cent._
Oak on Oak with Fibers parallel without lubricant 0.42
Oak on Oak with Fibers parallel rubbed with soap 0.16
Cast Iron on Oak 0.42
Cast Iron on Cast Iron, not lubricated 0.15
Cast Iron on Cast Iron, lubricated 0.10
Iron on Brass 0.16
Brass on Brass 0.20
Iron on Bronze, without lubricant 0.25
Iron on Bronze, thoroughly lubricated 0.06
Cast Iron Wheels on Rails (Rolling Friction) 0.004
Ball Bearings (Rolling Friction) 0.001