The friction between a car tire and the underlying asphalt drops off dangerously in rainy weather. Now, physicists in Germany and Italy have proposed an explanation for how even slight wetness can cut road-to-rubber friction.
Most, if not all, of the friction between a tire and a typical road has nothing to do with attractive intermolecular forces between the tire rubber and the road material, says theoretical physicist Bo N.J. Persson of the Research Center Jülich in Germany. Instead, by a mechanism dubbed bulk friction, the rubber of a moving tire briefly presses down into small pits in the road surface.
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