Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other.It opposes the motion and acts in the opposite direction to the motion.
Friction is produced between two surfaces in contact with each other because of tiny irregularities or projection these surfaces interlock and resist motion.
The frictional force between an object and a surface can be measured using a spring balance.
Is friction useful?
Friction prevents the surfaces from slipping past each other.Without friction we will not be able to walk,hold or write on a surface without slipping.There is more friction between two rough surfaces than between two smooth surfaces.
Friction produces heat.You can observe this by rubbing your hands together.The heat produced by friction can be put to use when lighting a match or starting a fire.
Friction slows down or stops the movement of object.Friction between air and a parachute(air resistance) slows down the parachute so that the parachutist does not plunge down onto the ground.Brakes in vechiles makes use of friction to slow down the vechile and bring them to a stop.
Is Friction Always Useful?
Sometimes,friction can be a nuisance.It slows down things unnecessarily.The friction between a boat and water reduces speed. Similarly the friction between the road and cycle reduces the speed and cyclist has to put more energy.
Machines feel warm because of friction between the moving parts which are in contact.These parts wear down with time due to friction.The efficiency of machines is reduced because of the energy which can be used to perform useful is converted to heat.The life span of machine is shortened because of wear and tear due to friction.
How to reduce friction?
To reduce friction,lubricants such grease and oil may be introduced between two surfaces.A cushion of air can reduce friction too,as in case of hovercrafts and maglev train.Ball bearings are used to reduce in moving parts.