Re: Conservation of ME and nonconservative forces

["QUIST, OREN" <OREN_QUIST@SDSTATE.EDU>]

The force does the work.  If you are interested in the work done by the hand
on the box, then you concentrate on the force of the hand on the box.  If
you want the force of the "left" part of the box on the "right" part of the
box, then you concentrate upon the force of the left part on the right part.
If you want the force done by friction, then you concentrate on the force of
friction.

If you want the total work done "on" the box, then you have to concentrate
on ALL of the forces acting "on" the box.  That is external forces, acting
on the box.  This is the case of the ball rolling down the inclined plane.
The total work equals its change in kinetic energy.  The forces doing work
are gravity, and friction.  The work done by gravity is measured by the
potential energy term, (-)mgh.  The work done by friction is zero if there
is no slipping.

Oren Quist, SDSU

-----Original Message-----
From: lorinda.stinnett [mailto:stinn001@BAMA.UA.EDU]
Sent: Tuesday, June 26, 2001 6:57 PM
To: PHYS-L@lists.nau.edu
Subject: Re: Conservation of ME and nonconservative forces


On Tue, 26 Jun 2001, QUIST, OREN wrote:

> Friction does no work in this case because there is no slipping.  That is,
> there is a force but no displacement of the contact point.
I agree with you.
My problem is that the displacement defined in work was not limited to the
point of contact.  For instance: If a box is pushed across the floor is
the work done only to the part of the box that is in contact with the hand
pushing it, or is the work done on the entire box?

> The problem is easily done using torques and Newton's Laws and the same
> answer is achieved.
Agreed!

> Oren Quist, SDSU
-Lorinda Stinnett