A ball rolling along the ground stops after moving through some distance. Similarly, a moving car begins to slow down the instant its engine is switched off and finally it stops.
According to Newton's first law of motion, a moving body continues to move along a straight line until unless an external force is applied on it. The ball or car is slowed down by a force called friction. Friction exists between the surfaces of all materials which are in contact with each other. The direction of the frictional force is always in a direction opposite to the motion.
If an object is to move with a constant velocity, a force equal to the opposing force of friction must be applied. In that condition the two forces are balanced forces. They exactly cancel one another and the net force on the body is zero. Hence the acceleration produced in the body is zero and the body maintains its velocity. It neither speeds up nor slows down.
The resistive force, before the body starts moving on a surface is called static friction. Once a body starts moving on a surface the friction between them is called sliding or kinetic friction. The sliding friction is slightly less than the static friction.
Smoothness of the surfaces is one of the factor on which friction depends. For example, smooth glass surface offers less friction compared to a rough sand bed.
Friction also depends upon the normal reaction. More force is needed to move a heavy box than to move a lighter box on the same surface. It is so because the heavy box has grater normal reaction (reaction of the surface on the box against the action of its weight) and hence greater frictional force.
The force of friction developed between the soles of your shoes and the ground helps us to move. Had there been no friction, walking or running would have been impossible.
You can write with a pen on page or with a chalk on the blackboard due to friction.
Buildings may be constructed only due to force of friction between different building materials. Without friction, you could not fix a nail on the wall.
Tyres of automobiles are treaded to increase the friction between tyres and surface of the road. Thus the tyres get better grip with the ground.
The breaks applied in automobiles also work only due to friction.
Due to friction, a lot of energy is wasted in the form of heat that causes wear and tear of the moving parts of a machine. Friction also reduces efficiency of the machines as considerable amount of energy is wasted in overcoming friction. However, the efficiency of a machine can be increased by putting a suitable lubricant between its moving parts.
In most of the machines, to reduce friction ball bearings are used between the moving parts. By using the ball bearing the sliding friction is replaced by rolling friction. As the rolling friction is less than the sliding friction, therefore, the friction between the moving parts is reduced.
Friction also wears out the soles of shoes. You would have seen that the steps of foot over-bridge at railway stations also wear out due to friction.