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At 10:14 AM 9/14/00 -0500, Jack Uretsky asserted:At 21:30 9/16/00 -0400, John Denker responded in a reasoned voice:
Sudden change of velocity implies infinite acceleration, which is infinitely
destructive and therefore unphysical. Sudden change of acceleration
does not violate any physical principle, although it may be difficult
to achieve in particular cirmumstances. So the displacement-time graph
need be only twice differentiable.
I'm skeptical of this argument, for dimensional reasons among others./snip/
Preliminary remark: One must always be careful with the meaning of
"infinity". In situations like this, the only safe procedure I know of is
to use it to denote the result of a limiting process, where something grows
without bound as we pass to the limit.
In this way I take Jack's statement to mean that there are physical
processes which in some limit can be extrapolated to give an unboundedly
sudden change in acceleration, i.e. an infinite jerk.