Friction force

When one body slides over the surface of another, sliding friction arises. Its magnitude is directly proportional to the normal force N pressing the surfaces together (the support reaction): F = μN. Here μ is the coefficient of sliding friction, a dimensionless quantity depending on the materials and surface finish and hardly on the contact area. On a horizontal surface N = mg. Rolling friction (a wheel or ball rolling on the ground) is much smaller than sliding friction in the same conditions, so it is easier to carry a heavy load on a cart than to drag it. Oil, wheels and bearings are used to reduce friction.