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Re: Work-Energy or Work-Kinetic Energy??



Hi,

I think it depends on how you define your system. If the entire
universe is your system then there is no work. ( No work, an inviting
idea!) If your system is the mass, then the external lift, and the
external gravity both do work. If the system is the mass and the earth
then the work associated with the lifting can appear in both KE and PE.

Thanks
Roger Haar U of AZ


On Thu, 6 Nov 1997, Timothy J. Folkerts wrote:


I just got into a discussio with a collegue on the interpretation of
the work-energy theorem, and we had quite different views on this basic
topic. Is it W(net) = Delta(E), or W(net) = Delta(KE)?

Consider a system consisting of a single mass being lifted:
View 1) You do work, so energy of the system increases. This shows up
as potential energy.
View 2) You do work, but gravity does equal negative work. The total
work done is zero, so KE stays the same.

Or consider the mechanical equivalent of heat, with the water as the
system.
View 1) You do work, so the energy of the system increases. This
shows up as a rise in temperature.
View 2) You do work, but friction does an equal and opposite work.
Thus the KE of the system is still the same.

So... Which interpretation is better?

Every introductory book I checked calls it "Work-Energy" (not
"Work-Kinetic Energy"), but they all write it as W(net) = Delta(K) (not
W(net) = Delta(E)).

I'm so confused! :)


--- Tim

---------------------------------------------------------
Timothy J. Folkerts email: tim.folkerts@valpo.edu
Department of Physics phone: 219-464-6634
Valparaiso University
Valparaiso, IN 46383