Re: Energy, etc

[Brian McInnes <bmcinnes@PNC.COM.AU>]

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Sarma,
you wrote
> 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.

For years I taught the work-kinetic energy theorem as a corner stone
of mechanics and it took some intellectual effort, aided by Arons'
book and discussions with Ian Sefton, a wise colleague of mine, to
realise that the real world was not a collection of individual
particles and that there were more valid models than the work-energy
theorem to explain it.

Brian McInnes