Friction is defined as a force acting opposite to the direction of relative motion. Friction may rise on the interface between bodies, but may also be within the body. Friction is not a fundamental force, but rather a manifestation of electro-magnetic and gravity forces.
In general, friction can be dry or lubricated. For most engineering materials, dry friction is larger than lubricated since the shear strength of the solids is typically larger. In classical lubrication theory, the friction is reduced by the introduction of the liquid lubricant (thin film lubrication theory). However, the theory has reached its fundamental limit (0.01-0.04 friction coefficient) and cannot be used to reduce the friction further. Therefore, the researchers look for other ways to reduce friction and recently several cases of superlubricity (friction coefficient well below 0.01) were reported (look further on tribonet).
Friction is a non-conservative force and the energy spent to overcome friction is lost. Friction losses reduce the energy efficiency of many mechanical devices. A recent report on the energy consumption due to frictional losses in passenger cars estimates that only 21.5% of the potential fuel energy is used to move the car, whereas direct frictional losses account for 33% (28% if breaking is excluded). Within these losses, 35% is used to overcome tire-road friction, 35% to overcome engine friction, 15% – transmission and 15% brake contact friction.
Leonardo da Vinci was the first to perform a systematic analysis of friction! He formulated the laws of friction, but his works were not discovered until recently. There are 3 laws of friction:
- Amontons’ First Law: The force of friction is directly proportional to the applied load.
- Amontons’ Second Law: The force of friction is independent of the apparent area of contact.
- Coulomb’s Law of Friction: Kinetic friction is independent of the sliding velocity.
Kinetic friction is the defined as the friction developed during relative motion, while static is the force developed prior the motion starts. Typically, the kinetic friction is smaller than the static.