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

[George Spagna <gspagna@rmc.edu>]

At 11:38 AM 11/6/97 +0000, Tim 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)?

It may be really a matter of definition.  In most applications I use the
latter, with the understanding that the net work must account for all
forces.  The former might be acceptable if one is specific in stating that
the work is done by an external force acting on a system.

> 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.

Yes, but this assumes that you are outside the system of mass plus Earth.

> View 2)  You do work, but gravity does equal negative work.  The total 
> work done is zero, so KE stays the same.

This includes your force and the force of gravity acting on the mass, which
is its own system.

> 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?

Again, where are you drawing the boundaries of the system?

> 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)).

Ah, so what's in a name?

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