Friction is the force resisting the relative motion of two surfaces in contact or a surface in contact with a fluid (for example air on an aircraft or water in a pipe). It isn't a fundamental force, as it's derived from electromagnetic forces between atoms and electrons, and so can't be calculated from first principles, but instead must be found empirically. When contacting surfaces move relative to each other, the friction between the two objects converts kinetic energy into thermal energy, or… (More on Friction)