Friction is a force that opposes the relative motion between two surfaces of objects in contact. The force of friction always acts in a direction opposite to that of the applied force. Friction is due to irregularities on the surfaces of the objects in contact. Friction depends on the smoothness of the surfaces in contact. The force of friction depends on the nature of the surfaces in contact. The force of friction increases if the surfaces are pressed harder. Frictional force that comes into action before the start of the motion of an object is called static friction. When an object slides over another surface, the frictional force that comes into action is sliding friction.
When a roller rolls over a surface, the frictional force that comes into action is rolling friction. Rolling friction is less than sliding friction, while sliding friction is less than static friction. Friction due to gases and liquids is called fluid friction, and is also called drag. Friction is a necessary evil. There are instances in daily life where friction is a necessity. For example, without friction, we cannot hold objects in our hands; we cannot walk and cannot light a match stick. Examples where friction has to be minimised and not desirable is the friction between machinery parts, which causes wear and tear.