Friction research casts doubt on fundamental physics law

New research on frictional slipping has revealed that some of the basic assumptions of introductory physics do not hold at small scales. The findings may be useful in the study of earthquakes.

A basic law of physics is that the force required to set an object moving equals the frictional force keeping the object stationary. The force from friction is determined by the coefficient of friction, the ratio of the sideways force to the force pushing down (the weight of the object). This relationship was first described by Leonardo da Vinci and further defined by Charles Coulomb and Guillaume Amontons centuries later.

Now a team led by physicist Oded Ben-David, a PhD student from the Hebrew University of Jerusalem studying the dynamics of frictional slips in carefully controlled laboratory experiments has found they could apply as much as five times more sideways force to the object than predicted by the coefficient of friction before the object would move.

The experiment’s set up, described by Ben-David’s supervisor Professor Jay Fineberg as the “stupidest system you could think about,” was to place two 200 mm Plexiglass blocks together and use tons of force to press them together, and then try to push the top block sideways until it began to move. They used sensitive strain gauges to measure all the stresses on the blocks and high-speed cameras and lasers to track the points at which the blocks actually touched each other.

The results showed that the blocks, which were optically flat and appeared to be touching all along their surfaces, were actually only touching at a few hundred contact points, and at each of these points the sideways forces could be much larger than predicted by the coefficient of friction before the contacts broke and the block started to move. When the contact was ruptured waves similar to sound waves were propelled through the blocks, resembling a mini-earthquake.

Professor Fineberg said the blocks represented tectonic plates sliding against each other. When the force is strong enough to pull them apart a series of shock waves result. The experiments were able to measure all the variables, which is impossible in a real situation. ....

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