Re: [Phys-l] Energy Question Negative Reps.

[John Denker <jsd@av8n.com>]

On 01/12/2007 02:58 PM, Bob LaMontagne wrote:
> If no net change in KE occurs from beginning to end, and no net change in PE
> occurs from beginning to end, then the lifter did no work on the barbell -
> pure and simple. Why is there a discussion on this? Are we making up an
> alternate definition of work?

We have long since been saddled with multiple inconsistent
definitions of work.
  http://www.av8n.com/physics/thermo-laws.htm#sec-work

The work/KE theorem applies to the overall work done on the object
by *all* agents.
  http://www.av8n.com/physics/kinetic-energy.htm
FWIW there are multiple versions of the work/KE theorem, but the
distinctions are not important here.

In this case there is positive work done by one agent and negative
work done by another ... lifter and gravitational field.  We can
talk about each of these contributions separately ... but we cannot
plug either one separately into the work/KE theorem.

> The fact that this doesn't jibe with our intuition probably just means that
> the work-KE theorem is probably not the clearest way to present physics to
> students.

The theorem -- if properly stated -- is still valid.  Alas applying it
to this problem isn't particularly rewarding, and doesn't address the
fundamental issues raised by the original question.