You might have noticed that a moving car begins to slow down the instant its engine is switched off. Why does it happen? In fact the car is slowed down by a force called friction, which exists between the surfaces of all materials which rub against each other.

8.3.1 Factors affecting friction

Friction is caused due to the irregularities i.e., elevations and depressions in the surfaces of sliding objects. These irregularities act as obstructions to motion. The direction of the frictional force is always in a direction opposite to the motion.
Thus, if an object is to move at a constant velocity, a force equal to the opposing force of friction must be applied. In that condition the two forces exactly cancel one another and the net force on the body is zero; hence the acceleration produced in the body is zero. But zero acceleration does not mean zero velocity. Zero acceleration means that 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.
So it may be concluded that force needed to overcome friction, is necessary to maintain the uniform motion of a body. Air resistance is one type of frictional force.
It is a common experience that it is difficult to walk on sand, but it is easier to walk on a metalled road. Greater the roughness, greater is the friction. More power is needed to develop same speed on the same road for a heavy truck than for a lighter truck. It is so because the heavy truck has greater normal reaction (reaction of road on the truck against the action or weight of the loaded truck) and hence greater frictional force.

8.3.2 Sliding and Rolling Friction

Once a body starts moving on a surface the friction between them is called sliding or kinetic friction. This friction is less than the static friction discussed above. You might have used a slide in your school or a park in your childhood to play with. Here force of friction is much less.You may be using a bicycle or a scooter to go from one place to another. The wheels are round and these roll over the road. Friction between wheels and road is rolling friction. This type of friction is least of all other types. Ball bearings do have rolling friction.