What is friction?« Back to Questions List

When two objects are in contact with each other, friction is the force resisting the motion of one of the objects relative to the other object. Friction arises due to surface imperfections and attractive forces that exist between the surfaces of the objects in contact.

There are many types of friction – static, kinetic, rolling, fluid friction. The first three types occur between solid surfaces and fluid friction occurs in liquids and gases.



Static friction is force that opposes any attempt to move a stationary object along a surface. For example, when we keep a book on a table and try to move it along the table. Kinetic friction arises when the objects in contact are in motion relative to one another. Rolling friction is the force acting on object that resists the motion of the object when rolling on the surface. 

Friction is what helps us to walk. When we try to walk, the following forces act on the person. Force of gravity acts on the person trying to pull downwards but still there is no acceleration downwards because there is a reaction force called Normal force acting upwards on the body. This force balances the body weight when we stand still resulting in no motion. When we try to walk, a force is transmitted from the foot to the ground as the heel touches the ground. As the heel strikes the ground, frictional force (F) between the foot and the ground, acts backward to prevent the foot from slipping. As the foot leaves the ground (meaning you are now pushing the ground backward), friction (F) acts in the forward direction to prevent foot from slipping backward and helps accelerate the body forward.

In the same way, friction also helps write on a paper and stop the car tires when brakes are applied. Thus frictional force is essential everywhere. 

One major concern about friction is that it causes wear and tear of machinery, automobile components etc. We add lubricants like grease, oil to all kinds of machines, automobile parts just to reduce the friction which in turn reduces wear and tear of the part involved in motion. 

Frictional force also converts a small percentage of kinetic energy into thermal energy. This means the moving parts of machinery or vehicles get hot after some time due to friction existing between them. This heat may in a long run ruin the components of machinery or automobile decreasing their lifetime. Also in overcoming friction, a part of useful energy gets spent.

But what would actually happen when there is no friction at all? To answer this question, imagine walking on very slippery surface of ice. Now all that you can do is fall down. Ideally there is friction existing everywhere.


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