FRICTION

The resistance to movement between any two objects when placed in contact with each other. Friction causes wear and heat. In an engine, it robs it of some of its potential power. Friction is not constant but depends on the materials, type of surface finish, amount of pressure holding the two objects together, and the relative amount of movement between the objects. Lubrication like oil, grease , and other materials like graphite reduce friction. Also see anti-friction bearing and coefficient of friction .