In principle. A body like a ring or a sphere rolling without slipping over a horizontal plane will suffer no friction 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 motion (rolling friction) does occur. Le to keep the body rolling, some applied force is needed. For the same weight, rolling friction is much smaller (even by 2 or 3 orders of magnitude). This is the reason why the discovery of the wheel has been a major milestone in human history.
Rolling friction again has a complex origin. Though somewhat different from that of static and sliding friction. During rolling, the surfaces in contact get momentarily deformed a little, resulting in a finite area (not a point) of the body in contact with the surface. The net effect is that the component of the contact force parallel to the surface opposes motion.
We often regard friction as something undesirable. In many situations, like in a machine with different moving parts, friction does have a negative role. It opposes the relative motion and thereby dissipates power in the form of heat, etc. Lubricants are a way of reducing kinetic friction in a machine. Another way is to use ball bearings between two moving parts of a machine. Since the rolling friction between ball bearings and the surfaces in contact is minimal, power dissipation is reduced. A thin cushion of air maintained between solid surfaces in relative motion is another effective way of reducing friction.
In many practical situations, however, friction is critically needed. Kinetic friction that dissipates power is nevertheless important for quickly stopping relative motion. It is made use of by brakes in machines and automobiles. Similarly, static friction is important in daily life. We can walk because of friction. A car can't move on a very slippery road. The friction between the tires and the road provides the necessary external force to accelerate the car on an ordinary road.
Let us return to the example of a body of mass m at rest on a horizontal table. The force of gravity (mg) is canceled by the table's normal reaction force (M). Now suppose a force Fis applied horizontally to the body. We know from experience that a small applied force may not be enough to move the body. But if the applied force F were the only external force on the body. It must move with acceleration F/m, however small. Clearly, the body remains at rest because some other force is horizontally and opposes the applied force F. This results in zero net force on the body. This force parallel to the body's surface in contact with the table is called frictional force, or simply friction. The subscript stands for static friction to distinguish it from kinetic friction f that we consider later.
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