Friction is the resistance which one body offers to another sliding over it. Additional work is required to overcome this resistance when it occurs in any mechanical system, thus increasing the power consumption and the energy needed. Considerable research and design effort is devoted to eliminating friction from mechanical systems, aircraft, vehicles, and the various forms of water craft.
It has been estimated that over 30% of energy generated is consumed in friction, manifest as the heating of the moving parts. In the particular case of the automobile, some 20% is so thus wasted.
While the presence of friction effects is a considerable obstacle to the increase in the efficiency of mechanical and transport devices, the complete absence of these same effects would create another set of difficulties, namely stopping or braking such devices once set in motion. Indeed, friction is a very useful and economical means to control the motion of mechanical parts or vehicles.