Rolling Friction:

While a body (for example a wheel) rolls on the ground, the point of wheel in contact with the ground at any instant, has no relative motion with respect to the ground. Therefore, a large amount of frictional force is removed. But in practice, the resistance to rolling motion exists. It is mainly because of the deformation of the surface upon which the wheel rolls. Therefore, the contact between wheel and the ground is not limited to a single point but extends over an area. The distance 'a' as shown in Figure is referred as, the 'forward length of deformation' and is defined as the coefficient of rolling resistance.