|MadSci Network: Physics|
Sorry, Steve. You did not provide enough information for a guaranteed answer to your quandary. But there still may be hope. Let us assume a situation. Say we are talking about a bicycle wheel, spun by hand. Unless you have faster hands than I, the air resistance will be negligible, so the friction will result primarily from the bearings and seals. Under these assumed conditions, friction will be nearly constant, at least over the range of motion that one can detect with normal instrumentation. Classical lubrication theory describes three regimes of lubrication. This example falls in the first category, called boundary lubrication, where friction is nearly constant with the lubrication parameter. This lubrication parameter is linear with speed and lubricant viscosity, but is inversely related to the applied load. Since the lubricant temperature is nearly constant, its viscosity also is nearly constant. The applied load in our assumed case is simply the weight of the wheel. Thus, for our assumed case, the lubrication parameter varies simply and linearly with speed. Higher operating speeds could lead to a second regime, called thin film lubrication. Here, friction drops with increasing speed as a full hydrodynamic lubricating film develops. Even higher operating speeds would lead to the third regime, called thick film lubrication. Here, the friction rises with increasing speed. What you were told was correct. Dynamic friction does depend on velocity. However, that does not necessarily mean that the dependence is large--at least over moderate speed ranges. The transition from static to dynamic friction is a bit stickier (pardon the pun). In the presence of some lubricants, static friction can be lower than low speed dynamic friction. With dry, unlubricated frictional contacts, static friction normally is higher. The last time I tried to study static-to-dynamic transition behavior, my patience was severely tested. At room temperature, some fatty acids (chemisorption agents) were found to be effective friction modifiers at speeds below 0.001 rpm. Several days of testing were required to map frictional transitions (from rest to 0.1 rpm) for these potent boundary friction modifiers. I doubt that your test would have detected this transition from dynamic to static friction. You may wish to look up a reference on friction, lubrication, and wear. Volume 18 of the ASM Handbook, Lubrication Regimes, is a good starting point. Not much was found on the Internet, but you might try the following site (watch out for their non-linear scales): http://squid.ucsb.edu/~sfalab/ProjectFrictionPhDia.html
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