Re: one question

[Steve VanDevender <stevev@efn.org>]

KellySt@aol.com writes:
 > >> > > Yes, in fact is does not take any energy to keep
 > >> > > floating a few metres above Earth's surface.
 > >
 > >> >Well, it depends on how you make the object float.
 > 
 > >> No, if you for example use a helicopter, it does not take
 > >> energy to stay at that height but it does take energy to
 > >> move all the air.
 > 
 > >> Timothy
 > 
 > Hold your arm out to you side.  Tell me it takes no effort (energy) to hold
 > it there.

Your muscles are not static structures, as I have pointed out
before.  They produce force with protein fibers that must
continually dissipate energy by moving molecules around to
maintain constant tension.  This constant chemical activity is
what you feel as effort when you hold out your arm.

Now nail a board at a right angle to a vertical post.  How much
energy does it take to hold it there?  None.  How much energy did
it take to put it there?  Some to lift the board and pound the
nails.  The motions required to do those actions performed work;
they involved application of force over a distance.  Some of this
work heated the nails and the board; once you are done pounding
no more energy is put into the nails or the board, and they cool.
Once they cool to ambient temperature no more energy goes into or
out of the board and the nails.  Gravity cannot change the energy
of the board because the board does not move through the
gravitational field.

That it takes effort for your body to produce a constant force
does not mean that all forces result from constant effort
(dissipation of energy).  It means that your body is not an ideal
machine.  A force that produces no motion dissipates no energy.