On 05/17/2003 11:38 PM, Bob LaMontagne wrote:
>
> Just a thought - place a crate on the back of a flatbed truck - then
> drive it around the block a few times and park it in the original
> location. Net work is zero and is path independent. Static doesn't
> necessarily exclude motion.
That's an amusing and informative Gedankenexperiment.
Let's take it one step further: Find or make a
convenient mark on the road. Drive over the
mark at 10mph while accelerating. Drive around
the block and drive over the mark again at 20mph
while decelerating.
The force is not a function of position.
The energy is not a function of position.
Conclusion: Saying something is "static friction"
is not sufficient to determine whether the
process is conservative or not. The situation
cannot be analyzed until we know what other
information and assumptions are involved.
More generally, saying that something is a "force
of constraint" tells us a few interesting, easily
interpretable things if the constraint is stationary...
whereas moving constraints generally require very
detailed specification and analysis.