ROLLING FRICTION
Rolling friction is of a different kind. The intermeshing inequalities or microscopic teeth are lifted out of contact with, one another just as the teeth of gear wheels are carried out of mesh. But there is another cause of friction due to the fact that no objects are so microscopic that they do not sink into each other to some extent. A wheel is always rolling out the surface it is turning on just as a rolling pin rolls out dough. If the surface is of elastic material such as a steel rail, it springs back into place immediately after the passage of the wheel, but the wheel must constantly travel in the trough of a wave which accompanies it along the rail. There is a similar wave in the wheel itself and this ironing-out action produces heat in the wheel and the rail. It is particularly noticeable in the flexible tires of automobiles which, after a run on even a smooth road, become too hot to be grasped with the bare hand merely because of the waves of compression and decompression to which they are subjected.
Both rolling and sliding friction are increased by pressure because the depression is greater and because inequalities are brought into more intimate engagement with one another. The degree of friction also depends upon the nature of the substances in contact, but theoretically the area of contact does not make any difference. It is just as hard to push a block along a smooth surface on its edge as on its side.