So, I know that many materials have different coefficients for static and dynamic friction. why? What makes a moving surface different from a stationary surface?
Logically, it makes sense, but I think that’s just because we’re used to handling objects large enough to have a noticeable inertia. However, AIUI, friction applies at all sizes, even ones where you wouldn’t expect inertia to be a factor (relative to material strength, friction, etc.), like two sheets of paper, for instance.
note: question inspired by the mythbusters segment Phone Book Friction
My guess has always been that, at rest, the two surfaces get a chance to nestle really close together, fitting crannies on one side into nooks on the other. Once motion is going, it doesn’t get a chance to settle down so securely.
At the very least, it’s trivial to show that, if static and dynamic friction are both given by f = mu*N, then mu_static must be greater than or equal to mu_kinetic. Think about it for a bit, and you’ll see why.
If mu_static were less than mu_kinetic, and you applied a force greater than mu_staticN but less than mu_kineticN, then it would not be possible for the object to be either moving nor stationary.
Thats all sorts of factors, but the movement is pushing one surface up away from the other and making the normal force push less and making less contact between the two surfaces too.
Since no one has given a definitive answer, I will chime in with my WAG. I think static friction is based on (usually) weak chemical bonds between the two surfaces. When sliding, they are not in contact long enough for the bonds to form. When a surface is chemically inert (e.g. teflon) there are few, if any, bonds and the difference will be small.
If what you’re hypothesizing is true, then the box moving with a speed of 1 mm/s across the floor would have higher dynamic friction that the box moving with a speed of 3 mm/s across the floor.
Is this the case?
What I’m getting at… at of 1 mm/s, there should be ample “time” for the molecular attraction to become significant. At 3 mm/s, there’s not as much time available, and thus the forces due to molecular attraction would be less.
Friction is the force that needs to be exceeded before the object will move. With inertia, the object will accelerate (maybe very, very slowly) regardless of how small a force is applied to it.
This is sort of correct in the same manner that water is a beverage, but not the only thing available to you at the bar. What we refer to as “dry” friction (e.g. the force opposing the direction of motion observed between unlubricated solid surfaces in proportion to the normal force between them) is actually an amalgam of different effects that the molecular or atomic level as well as potentially on a larger scale with rough surfaces. The largest set of effects in dry friction is due to van der Waals forces, which are the weak intermolecular forces. (It would be a mistake to call these bonds as they aren’t really the result of a discrete sharing or granting of electrons but are rather due to polarization and field effects between molecules.) The mu values you find for static and dynamic friction aren’t developed from any kind of first principles analysis; they’re purely empirical measurements which tend to vary widely depending on surface finish, thermal condition, and other effects. And while they are treated as a single value for dynamic friction, the actual frictional effects do vary with speed. There are also other contributors such as mechanical interference or locking (microscopic or larger geometric interferences), viscous drag (friction due to the motion of an interstitial fluid or pseudofluid), adhesion (attractive chemical bonds that require zero normal force to restrain motion), net negative pressure (the interstitial pressure is lower than the ambient, resulting in an inward directed force), and paramagnetic effects such as eddy flow. It is even possible in certain (very limited) circumstances to have a negative coefficient of friction such that friction is reduced by applying more force.
The force of inertia is very simply F[SUB]i[/SUB]=m╳a. Since the acceleration at the static condition is essentially zero and the net acceleration at a constant velocity is also zero, the force due to friction can be found by direct measurement. Finding friction under acceleration is a little more complicated; the dynamic friction is assumed to be the same regardless of speed (and generally speaking is close enough) so under steady acceleration you can subtract F[SUB]i[/SUB] from the total force to get friction. In laboratory measurement, many trials are run on identically prepared specimens with a range of normal forces and a statistical range is determined from the data. With true dry friction (no lubrication of any kind) it is common to find a fairly wide range of variability (ten percent or more) for most materials even with a very uniformly prepared contact surface.
Thanks for the info all. Sounds like this is one of those things that’s rather complex, and thus, not really well understood.
I do understand now, why they have to be different. I hadn’t really looked at the formula before, just dug up the values for various common materials to make sure it wasn’t just one class of materials where they’re different.
The newest understanding of friction seems to say that it is much less the actual surface roughness caused by tiny nooks and crannies but an atomic level phenomenon, with friction increasing with pressure due to more of the surface of the objects being in contact atomically.