Re: A few silly questions

On Feb 14, 12:18 am, "David L. Burkhead" <dburkh...@xxxxxxx> wrote:
Ben Newsam wrote:
On Fri, 09 Feb 2007 19:56:24 GMT, "Wayne Dobson"
<nos...@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.

But we could do a ballpark estimate. It would be based on
assuming purely vertical forces, an estimate of the thickness of
the rope used and a guess as to its material (these lead to an
estimate of the spring constant). From an estimate of the
spring constant you would be able to work out the stopping
distance and time.

And as you say, the stopping distance or time for a given
momentum is what tells you the force involved in that stopping.
Specifically, F = dp/dt. You can also work it out in terms
of energy and distance: work = integral(F*x dx) = 0.5*mv^2.

If you are driving a car at 60 mph, you can stop in a long
distance and time, as you do when you press the brakes,
or you can stop in a short distance and time, as you do when
you hit a wall. The forces in the second case are higher.

[snip remainder of argument]

- Randy

.

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