Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: PHYS-L Digest - 13 Sep 2002 - Special issue (#2002-406)

Like Carl, I thought I understood John Denker simply to be making
another of his principled stands against characterizing what many of
us like to call the "pseudowork-kinetic energy theorem" a "proper"
work-energy theorem. But John says he isn't arguing over semantics
or names which leaves me quite mystified and even more curious.

So John, if you are really claiming that the change in the bulk
translational kinetic energy of the block as it slides to rest on the
table is *not* equal in magnitude to the product of the frictional
force on the block and the distance the block travels, could you give
us your best estimate of how they *are* related? (In particular, why
the rough factor of two?) And if that is not what you are claiming
could you be a little less coy about the point you *are* trying to
make?

John Mallinckrodt
Cal Poly Pomona

"Carl E. Mungan" wrote:

Free-body diagram. The net force on the block is -umg. This equals
ma. Thus a = -ug which is a constant. Thus we can use the equation of
kinematics, v^2 = v0^2 + 2ax. But v = 0.

I recognize that as an equation from point-particle
kinematics. If you claim it applies to objects with
nontrivial internal structure, please provide a
derivation or a pointer to a derivation.

Hint: I've got a pocketful of counterexamples, so
any such derivation is going to be quite a rare bird.

We insist there is a logically sound theorem
applicable to the block as a whole.

I'm from Missouri. Show me.

We can agree there is a problem
of what to *name* various items.

I am not interested in quibbling about names.
I haven't raised the naming issue.
I agree we shouldn't go there now.

That's fine and I have no problems with your personal preferences.
"De gustibus non disputatum."

Since when is the definition of kinetic energy a
matter of personal preference? The attempted
calculation in the previous note miscalculated
the kinetic energy by roughly a factor of two.

I don't see how a logically sound theorem could
produce such a result.