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Re: Friction



2. Max acc is always achieved with some slippage of the tires. Therefore,
slipping friction must be greater than static friction. Drag racers do not
spin their tires because they are out of control. They do it because it gives
the max acc. Likewise a race car in a turn is deliberately set up to slide.
Sliding is faster. To stop the slide you have to slow down!

If I read the above paragraph correctly, am I to conclude that slipping
friction (kinetic) is always greater than static friction? Balderdash.

small and my measurements were not that accurate. Has anybody experimented
with motion detectors and force probes connected to a computer or graphing
calculator to look for these effects? My guess is that with their accuracy
it should be possible to show that at least for some materials our simple
linear models are not a very good approximation.


I have my students pull different masses across a table surface with a
calibrated force probe. The students will pull the mass at a speed as
constant as possible, then stop, and pull again. There are definite
spikes (static friction) and the kinetic friction (sliding) is much
lower. If the blocks are allowed to rest on the table for a period of
time, the first spike is much higher so we just average the spikes after
the first one. They are all nearly the same. There is a direct
relationship between the pulling force and the mass of the blocks.

We wrap the blocks with paper to provide the same surface. The bottom
block is turned on edge to provide a smaller surface area. Within the
limits of uncertainty, the force is independent of the surface area but
the results are less than convincing for most students.


Lowell