# Friction research casts doubt on fundamental physics law

##### October 11, 2010 by Lin Edwards, Phys.org report

(PhysOrg.com) -- 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 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 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 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.

The shock waves are of three types: slow ruptures, moving well below the speed of sound, sub-Rayleigh waves traveling along the surface at the speed of sound, and supershear waves, traveling fast enough to cause a sonic boom. The type of wave produced depends on the stresses on the contact points, and the experiments showed that even small nuances could change the dynamics.

The most common type of wave in earthquakes is the sub-Rayleigh type, and there is some speculation that an in Izmit, Turkey in 1999 was a supershear type.

The results of the research, published in the journal Science may have implications for engineering and materials science, and could add to the scientific understanding of how earthquakes occur. Fineberg said it might also be possible in the future to trigger small earthquakes to prevent larger, more devastating quakes.

More information: The Dynamics of the Onset of Frictional Slip, Oded Ben-David, Gil Cohen, Jay Fineberg, Science 8 October 2010: Vol. 330. no. 6001, pp. 211 - 214. DOI:10.1126/science.1194777

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##### TAz00
not rated yet Oct 11, 2010
Ahem, super high speed camera anyone?
##### Nik_2213
1 / 5 (1) Oct 11, 2010
With perspex, they may be able to use polarised light, too...
##### MorituriMax
1 / 5 (1) Oct 11, 2010
Seems to me that the key sentence is "were actually only touching at a few hundred contact points,"

Is the fundamental physics of friction still right, they are just getting more accurate indications that under real world conditions, using real world materials you just don't get perfect results in your tests?

It is nice to know that you do get these results which show that it isn't as cut and dried, that we have some wiggle room for technological innovations.
##### axemaster
5 / 5 (2) Oct 11, 2010
"introductory physics do not hold at small scales"

And I should be surprised... why?
##### chandram
3 / 5 (2) Oct 11, 2010
The physics involved is classical for the frictional theory. As we go on reducing the actual contact points between two large surfaces involved in spite of being optically plain, the Physics changes at such micro-level of actual contacts. It is an interesting observation which requires careful repetition and a detailed study when the break down takes place!NN
##### DamienS
5 / 5 (1) Oct 11, 2010
I remember reading long ago that the precise physics of friction were poorly understood - I guess not much has changed.
##### dan42day
3 / 5 (4) Oct 12, 2010
The coefficient of friction is an average for an area. Don't we all know intuitively, at least in a solid, that it actually involves forces able to cause deformation at the relatively few points that interfere?
##### tkjtkj
not rated yet Oct 12, 2010
Fineberg said it might also be possible in the future to trigger small earthquakes to prevent larger, more devastating quakes.

Wow! Yes, i really *should* have gone to law school! What a jungle of liability suites would there be!