Re: Work-energy worries

[John Mallinckrodt <ajm@CSUPOMONA.EDU>]

On Mon, 7 Oct 2002, Lemmerhirt, Fred wrote:

> I am about to assign the this problem:
>
> A 75kg boater tosses a 5kg anchor horizontally, straight forward across the
> bow of his 125kg boat at 2m/s relative to the water.  a) Neglecting any
> horizontal force applied to the boat by the water, calculate the boat's
> backward speed across the water just as the anchor leaves the boater's hand.
> (The boat is initially at rest on the water.)  b) At least how much work
> must the boater have done to throw the anchor in this way?
>
> But all of the PHYS-L debate about proper usage of work and energy
> terminology that I have followed has caused me to worry (possibly more than
> I should) about items like part b of this problem.  Would some object to
> this sort of question?  (The expected response is that the work should equal
> the total KE aquired by the boat, boater, and anchor.)

Well.  At the risk of fulfilling JD's prophecy allow me to suggest
what I think is a more defensible answer: The (minimum) work done by
the boater is equal to the total (bulk) kinetic energy acquired by the
boat and the anchor.  I get 10.16 J rather than the 10.25 J that is
obtained by adding in the boater's kinetic energy.  After all, who
among us--other than, perhaps, myself (and I'm not so sure about
that)--is flexible enough to accommodate the notion that the boater
does work on herself.

If the boater doesn't do work on herself, where *does* the boater's KE
"come from"?  I'd be more willing to accept at least the following two
answers:

1. It comes from herself via an energy transformation (from internal
form to bulk kinetic form) that is mediated by external forces
provided by the anchor and the boat.

2. It comes from pseudowork performed on the boater by the
anchor and the boat.

John Mallinckrodt
Cal Poly Pomona

P.S.  It's interesting to note that, to two or three sig figs, the
answer is the same whichever choice you make.  If I gave the problem
to students, I think I'd adjust the numbers to make the inclusion or
exclusion of the boater's KE have nonnegligible consequences.

This posting is the position of the writer, not that of SUNY-BSC, NAU or the AAPT.