I’m suffering muscle twinges in my neck and shoulder that are usually linked to excessive typing. As I have a grant proposal to review, a senior thesis to help whip into shape, and a book under contract, this means that blogging will be substantially reduced while I ration my typing to those things that pay the bills. You’ll get more linking and less thinking, at least until my shoulder calms down a bit.
I don’t want to pass up a set of links– two press releases and a news story— on some new results regarding friction:
If you want to reduce the friction between tiny objects, just increase the mass of the atoms at one surface, say researchers in the US. Heavier atoms vibrate at lower frequencies than their lighter couterparts, which the team has shown reduces the energy lost as heat as two materials rub against each other. The result could be useful for designing nanomaterials with specific frictional properties and may even provide a better fundamental understanding of friction — something that is lacking today (Science 318 780).
Friction between two sliding objects involves the conversion of kinetic energy into heat, which is essentially the vibration of atoms that make up the materials. Robert Carpick of the University of Pennsylvania and colleagues have gained new insight into how this conversion occurs by sliding an atomic force microscope (AFM) tip along single-crystal diamond and silicon surfaces. They measured the force of friction between the tip and surfaces covered by either a single layer of hydrogen or deuterium atoms. Deuterium has the same chemical properties as hydrogen but is twice as heavy – allowing the team to study the effect of atomic mass on friction without having to worry about chemical effects.
Friction is pretty much the bane of physics faculty.
Not only does it cause all sorts of problems with experimental apparatus– if it weren;t for friction, we wouldn’t have vacuum pump oil all over the place– but it’s a mess theoretically. Including friction turns simple physical situations into intractable problems requiring numerical solutions. And, on top of that, the standard model of friction that we teach to intro mechanics students is a really crude approximation, as anybody who has ever supervised a lab on the subject can tell you.
As a result, it’s nice to see some investigation of friction on a microscopic scale, to try to get a better handle on exactly what’s going on. These results aren’t terribly shocking, but it’s a fairly clever experiment to get at things that seem fairly obvious theoretically.
I also have some affection for the microscopic picture of friction, as it’s the one area where I use my rugby-playing experience in class. One of the areas that students always find confusing is the distinction between “kinetic” frictional forces on moving objects, and “static” frictional forces on objects that haven’t started moving yet. Static friction is generaly stronger than kinetic friction, but it only acts up to a point, and then lets go, which is why it’s hard to start sliding a heavy object across the floor, but easy to keep it going once it’s moving.
I explain the difference in class as being due to the microscopic roughness of two surfaces in contact with one another. Small bumps and projections can collide and snag on each other, and that leads to an effective force opposing the motion.
The rugby connection is this: when I played rugby back in college, the pitches we played on were about as far from the center of campus as you could possibly get, past the practice fields for all the other outdoor sports. As a result, nearly everyone drove down to practice, and being college students, we drove like maniacs.
The college didn’t like that very much, so they put in a couple of sets of speed bumps. They failed to appreciate, however, that there are two ways to deal with speed bumps: there’s the approach that the designers intend, where you go very slowly, and the wheels thump down into the gap between the bumps, and there’s the approach favored by deranged rugby players driving other peoples’ cars, which is to hit the bumps at 50 mph, in which case you skip lightly across the tops of the bumps, and never really fall all the way into the gaps.
Static friction is sort of like the traffic engineer approach: when you’re moving slowly, the microscopic rough patches have a chance to settle in and lock together, and breaking them apart to set the objects in motion requires a fair amount of work. Kinetic friction is like the rugby player method: if you’re moving fast enough, the rough bits bounce right over one another, and reuires less force to keep going. In car terms, the overall ride is smoother (though it does some violence to the suspension).
(The current results really don’t have anything to do with this model– they’re using a more sophisticated treatment of the surfaces than the atom-scale Velcro model I talk about in class– but I like the analogy, so I’ll use it here…)