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Rolling Friction

المؤلف:  GEORGE A. HOADLEY

المصدر:  ESSENTIALS OF PHYSICS

الجزء والصفحة:  p-112

2025-11-08

385

If two equal masses of iron are drawn over a smooth iron surface, one being in the form of a block with a flat base and the other in the form of a cylinder so arranged as to roll, it will be found that the cylinder offers much less resistance to the motion than the block does. Rolling friction depends upon the hardness and smoothness of the surfaces in contact. When these are very hard and smooth, rolling friction is much less in amount than sliding friction - as in the case of a car wheel running on a steel track. If, however, the surface over which a wheel rolls is soft and yielding, as in the case of a wagon in deep sand, rolling friction may be even greater than sliding friction. In such a case the wheel has to be constantly climbing the hill caused by the sinking of the wheel in the sand. If the wheel is yielding, as in the case of an automobile tire that is not well filled with air, the wheel is flattened at the point of contact, thus increasing the rolling friction.

The efficiency of machines is increased by changing sliding friction to rolling friction, by the use of hardened steel cylinders or balls placed between the axle and the bearing. In roller bearings, the contacts are line contacts, while in ball bearings, they are point contacts.