Re: Energy, etc

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

 Jim Green wrote
 "One does not have to go that far: There _is_ an
> > effective force on the CM.
> > The wall pushes on the finger bones; the finger bones push on the
> > wrist
> > bones; the wrist bones push on the ulna, the ulna pushes on the
> > humerus,
> > the ... "
I wrote
> > All true, Jim, but irrelevant to the question of whether or not an
> > external force is doing work on the person and thereby causing a
> > change in kinetic energy.

You wrote
> Not irrelevant! The person moves!  The hand does not move and no work is
> done on the hand.  But the elbow moves so work is done on the elbow -- by
> the ulna ... etc  -- at some point work is done on the CM bone and the
> person moves.
>
> Yes AND the Earth moves and work is done on the Earth!!!  AND therefore
> work is done on the person by the Earth!!!!!
>
> IE The wall moves!

But, Jim, the calculations of work and the implied recourse to the
concept of work as the explanation for the (change of) motion is
certainly unnecessary and, therefore, irrelevant to the explanation of
what is going on. What is needed to change motion is force.

As to the question of any work done by the earth on the person, let's
eliminate the wall and look at the simpler situation of a person
walking along a level path.  We could look at walking at a steady pace
but let's take the situation when the person starts walking from rest.
 In addition to the gravitational force and the normal component of
the contact force there is now a tangential (horizontal) component of
that contact force exerted by the ground (earth) on the walker.  There
is an equal and opposite force exerted by the walker on the ground.
That interaction is called the frictional force.  The point of
application of that force does not change as the walker moves forward.
 No work is done on by the ground (earth) on the walker and no work is
done by the walker on the earth.
Quite a number of internal forces will be acting in the walker and it
is a very interesting problem in bio-mechanics to track them down.
None of them do any work on the walker.
It is apparent that the walker's kinetic energy increases.  What is
the source of this increase? Muscular contractions and extensions -
associated with some of those many internal forces - have resulted in
a decrease of the body's chemical energy.  Note that not all of this
decrease in chemical energy leads to the increase in kinetic energy.
We notice the walker "warms up". So there is also an increase in
thermal energy that is part of the reckoning to be done in looking at
conservation of energy.  After a while there will be heat transfer to
the surroundings.  We can make quite a lot of use of the first law of
thermodynamics here for the system we are calling a walker but in all
of it the value of (delta W) is zero.

Brian McInnes