When a solid object mives over the sruface if another solid object， its motion is always opposed by a retarding force ，called Friction. Hence friction is an opposing force that come into play when one body moves or attempts to move over the surface of another body.The following are methods are used to reduce friction;-
(1) By use of Lubricants; When a machinery is working， many moving solid parts come to direct contact. By use a lubricant e.g oil or grease， a thin layer of lubricant is formed between the solid parts. Since friction between two liquid layer is much smaller than that between two solid surface， the use of Lubricants reduxe the friction considerably.
(2) Polishing; Polishing means to deposit a fine layer of the materials over the surface of the body. This layer covers the irregularities of the surface. Consequently， friction is greatly reduced. The same result can be achieved by rubbing the surface qith sand paper or emery powder.
(3) Proper selection of materials; the friction depend upon the nature of the materials. We normally use materials which form low friction combination. For example， machine bearing are made of one metal (e.g bronze) while rotating shaft is made from other metal (e.g steel). Similarly， tyres are made of rubber because rubber rod forms a low friction combination.
(4) Use of ball bearing， In rotating machine， the shaft are mounted on ball bearing to reduce friction. It is because rolling friction is very small as compared to sliding friction. The free wheels of cycles， the axles of motor cars and shaft of motor cars are provided with ball bearing.