Friction or frictional force is the force that acts between two surfaces in contact. When the surfaces of two bodies are in contact with each other, a force is required to move or slide one of them over the other. This is because there is usually another kind of force, which acts to oppose or prevent the movement of one body over the other.

This opposing force is known as friction and its magnitude depends on the nature of the surfaces of the bodies in contact.

Friction can be defined as a force, which acts at the surface of separation of two bodies in contact and tends to oppose the motion of one over the other.

Friction is an electromagnetic force, generated by the charged particles of the materials in contact. As an electromagnetic force, the value of friction cannot be determined from first principles or postulates, and so cannot be derived from calculations.

Friction can only be calculated by empirical means, which means it requires information obtained from experience or experiment to be able to calculate the friction between two surfaces in contact with each other.

In the process of the surfaces of two bodies rubbing against each other, energy conversion usually takes place. The friction acting between them changes kinetic energy into thermal or heat energy.

Types Of Friction

There are two types of friction, namely static friction and kinetic friction.  Static friction, also called limiting friction takes place when there is no motion, while kinetic friction, also known as dynamic or sliding friction occurs when there is motion.  

Laws Of Friction

The laws governing solid friction can be summarized as follows: 

1. Friction opposes motion.
2. Friction is independent of the area of the surfaces in contact.
3. Friction depends on the nature of the surfaces.
4. Friction is independent of relative velocity between the surfaces.
5. Frictional force F is proportional to the normal reaction R, exerted by the surface of the material on which a body rests.

 
As shown in the diagram above, the normal or perpendicular reaction R exerted by the surface on the body A is equal and opposite to the weight W of A.

                                    Therefore, R = W = mg

From experiments, it is found that when R is increased by adding more weights to A, the static frictional force, F also increases as the minimum force P needed to move A increases.

Therefore, the static frictional force, F is proportional to the normal reaction R.

F  α   R

 F = µR

      = µmg

  or  ^F/_R = µ

F = µR 

or F = µmg 

                   is the friction equation or formula.

See more friction topics here.
See coefficient of static friction here.
See coefficient of kinetic friction here.
See calculating friction here