Re: A few silly questions

Ben Newsam wrote:
On Fri, 09 Feb 2007 19:56:24 GMT, "Wayne Dobson"
<nospam@xxxxxxxxxxxxx> wrote:

By the way, I'm not just asking these questions
because I'm too lazy to do any study, but that without any outside
input, I get the feeling that the person I'm arguing with feels at
liberty to call anything I say, wrong, regardless of its merits.

We get a few of those in here.

Unfortunately, Mr. Dobson is one of them. He only came here looking for
"ammo."

I'm the person he was arguing with. The context was from his claim that, in
the case of a falling body brought to a stop by a cord (as in a hanging),
"considering only those two factors [height and weight], the forces involved
are very predictable." I immediately pointed out that those two factors
don't define the problem, let alone lead to "very predictable" forces.
Given only those two factors, the only thing that can really be said is 0 <
F < infinity. And since F here is magnitude rather than vector force,
that's really no restriction at all.

When we went around on this, the infinite force limit was brought up and Mr.
Dobson said that of course that would require infinite energy (or words to
that effect). Which I disputed since, yes, you would have infinite force,
but, since (by the initial stipulation of the condition), it would take
place over zero distance. infinity * zero, undefined but possibly
meaningful if considered as a limit. (In this case, the limit should come to
simply the kinetic energy of the falling body--not the "infinite energy" Mr.
Dobson claimed.) This, of course, leaves aside that stopping a moving body
in a particular frame doesn't require energy. Rather it gives up energy
whether it's used to move something else or is given up as heat, breaking
things, whatever.

Several times over the course of the discussion I pointed out that the
infinite force--where the time over which it is applied and, by extention,
the distance over which it is applied, is zero--was the case "at the limit."
Likewise the F=0 end (where the distance over which it is applied would be
zero. These were only cited as the extreme cases, the limits, which
illustrate why the claim of "forces are very predictable" is incorrect.

And, at the limit (speaking purely mathematically) there is no prohibition
on having a change in velocity/momentum occur in zero time. All you've done
is specify a point in the position/time curve where the curve is continuous
but at which the first and all subsequent derivatives does not exist.

Well, eventually, he finally got around to claiming that he actually meant
to include those other factors necessary to define the problem and we should
just try to forget that he spent considerable time trying to defend the
original proposition, as originally worded.
--
David L. Burkhead "Dum Vivimus Vivamus"
mailto:dburkhead@xxxxxxxxxxx "While we live, let us live."
My webcomic Cold Servings
http://www.coldservings.com -- Back from hiatus!
Updates Wednesdays



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