Re: Energy, etc

[Gordon Smith <slipstk@OLEMISS.EDU>]

> > Have you cosidered force acting on the hand due to the
> > wall as a function of time and can we be sure that there is no
> > displacement of the hand during this interval.Interaction always
> > involves finite intervals and in such a finite interval there is
> > always displacement.
>
> It is true that the force acting on the hand increases and then
> decreases. So there is a small change in the point of application of
> the contact force - a change which is far less than the displacement
> of the centre of mass of the person.  Let's look more closely at this
> small change of the point of application. What is happening is that as
> the wall deforms slightly the point (it's really a small region, not
> an abstract point) moves into the original position of the wall and
> then back to where it was originally (unless the guy has pushed so
> hard on the wall as to permanently deform it). No net work is done by
> the normal force exerted by the wall.

Um, I agree completely with what has been said so far.

The wall doesn't move, so no work
The hand pushing against the wall doesn't move (on a realistic scale) so no
work
The wrist attached to the hand pivots, so some work
The forearm attached to the wrist act as a lever, negligible work
The elbow pivots, so some work
The shoulder pivots, so some work

Linking all these together, we find that the whole arm system changes it's
effective horizontal length while pushing, so the reasonably constant force
that is applied to the wall can be said to be applied through that change
in distance.  Thus, as we push away from the wall, we are doing some work.
It's a question of frame of reference.

If you don't think this is so, place your hands attached to fully extended
arms on the wall, and push.  You'll probably find that you don't go as fast.

Forest for the trees, guys, forest for the trees...

-G-
-----------------------------------------
Gordon Smith
National Center for Physical Acoustics
Coliseum Drive
University, MS 38655
slipstk@olemiss.edu