why rolling friction is smaller than sliding friction?

Dear student,We know that the friction is the force that resists the motion of one surface relative to another with which it is in contact. When we slide an object over another, there is friction acting between the two surfaces which reduces the relative velocity between the two surfaces during sliding. Now, if we somehow reduce the area of contact between the two surfaces then we can reduce the effect of friction between the two surfaces drastically. Ideally, a body like a ring or a sphere rolling without slipping over a horizontal plane will suffer no friction. At every instant, there is just one point of contact between the body and the plane and this point has no motion relative to the plane. In this ideal situation, kinetic or static friction is zero and the body should continue to roll with constant velocity. We know, in practice, this will not happen and some resistance to the motion (rolling friction) does occur, i.e. to keep the body rolling, some applied force is needed but this friction (rolling friction) is much less than the sliding friction. This is the reason why for the same weight, rolling friction is much smaller than static or sliding friction.Regards