Difference between revisions of "Amontons' first law of friction"

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Revision as of 12:23, 24 June 2009

Statement

The friction force experienced by two dry bodies whose planar surfaces are in contact is independent of the area of contact between the bodies.

Amonton's first law of friction is implicit in the Coulomb model of friction, and is closely related to Amonton's second law of friction, which says that the friction force is (limiting value for static friction, actual value for kinetic friction) proportional to the normal force.

Apparent counterexamples

Surface area and rolling

One might initially suppose that rolling wheels is easier than dragging boxes of the same material because the surface area of contact of a wheel with the ground is less. This is, however, not the real reason. In fact, dragging a wheel is as different as dragging a box of the same weight. The reason why rolling a wheel is easier is because during rolling, there is no actual slipping of the contact surfaces against each other, so friction does not oppose the rolling. In fact, friction helps with rolling.

(There is another force, called rolling friction, which is of much smaller magnitude and does oppose rolling. This is a component of the normal force that acts to oppose rolling because of minor deformations of the surface).