Archive for September 10th, 2008

The third Giant’s Shoulders: five days to go

September 10, 2008

As noted earlier, the third Giant’s Shoulders carnival will be hosted here on the 15th of this month; so, send your entries to me at this email address (preferably on or before 14th of September 2008):

gs dot carnival dot guru at gmail dot com

I am looking forward to a carnival with huge number of entries which will keep the readers occupied for at least a month. So, send as many entries as you can and on as varied subjects as you can.

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Static friction is not truly static!

September 10, 2008

Martin H Mueser tackles the question How static is static friction in the latest PNAS:

The observations by Yang, Zhang, and Marder certainly further solidify the idea that static friction is not truly static and it is intriguing that their observations can be cast into a simple and thus elegant rate-state model, which would require interfaces to slip before they stick. Still, it will not be necessary to rewrite all aspects on solid friction in physics 101 textbooks. Velocities of 10−5 μm/s can be considered negligible for most practical purposes and it will remain easier to push a cruise ship through still waters than to slide a salt shaker over the dining table at such small velocities.

Hueser’s piece is a commentary on the accompanying paper of Yang, Zhang and Marder (referred to in the quote above): Dynamics of static friction between steel and silicon:

We conducted experiments in which steel and silicon or quartz are clamped together. Even with the smallest tangential forces we could apply, we always found reproducible sliding motions on the nanometer scale. The velocities we study are thousands of times smaller than in previous investigations. The samples first slide and then lock up even when external forces hold steady. One might call the result “slip-stick” friction. We account for the results with a phenomenological theory that results from considering the rate and state theory of dynamic friction at low velocities. Our measurements lead us to set the instantaneous coefficient of static friction that normally enters rate and state theories to zero.

Interesting piece; take a look!