Mechanical, chemical and heat energy

[Truman Prevatt <truman.prevatt@netsrq.com>]

Susan,

If I remember correctly you stated that KER and done research indicating
that a 500 kg horse requires the same amount of energy as a 400 kg horse
with a 100 kg rider.  Any freshman physics student could have told you
that. That is not a susprise.  

I would like to know how the weight was connected and how it changed the
center of mass of the horse from it natural center of mass.  If it
weren't a rider but attached weight which didn't impact the horse's
center of mass in the same way as the rider would, then the results
cannot be used to imply that for a rider.  When the center of mass of
the horse is significantly changed then the efficiency of the pullies
and levers the horse uses to propell itself could be dimished.  The
physics is subitle but there is a difference in the following
experiments - a 400 lg horse with a 6 foot 100 kg rider, a 400 kg horse
carrying weight spread ofer a large area on and around the back but with
all the weight within six inches of the horse vs. a 500 kg horse. To see
this pack an efficient 50 backpack, i.e. weight near your center of
gravity.  Then pack the weight so that most of the heavy items at at the
top of the pack.  It requires more work to carry this "unbalanced" pack
because the body is not designed and tuned to carry the perturbed center
of mass.

Energy is conserved.  If it takes X cal to mover a 500 kg horse or a 400
kg horse with a 100 kg rider, then Y cal of heat have to be dissipated. 
A 500 kg horse has 25% more surface area from which to dissipate this
excess heat.  Hence in conditions where heat dissipation is compormised
such that the 500 kg horse was at its limit of heat dissipation, the 400
kg horse will be at a serious disadvantage.  

I believe any study, to be comprehensive enough, to be of PRATICAL use
needs to take into account the entire engery chain - the mechanical work
required (easy to calculate since gravity, the primary force, is
conservative), the chemical enqergy + heat energy requred for this work
to be done and the dissipation of the heat energy.  The heat energy
being from the chemical reactions to produce the mechanical energy which
results from an efficient conversion between chemical and mechanical
energy.  This has to be then balanced with the heat dissipation side of
the equation.  I believe that ignoring this issues in the design of
experiment is ignoring a significant factor that must be considered to
get a realistic picture.

Truman

-- 
Truman Prevatt
Mystic “The Horse from Hell” Storm
Rocket a.k.a. Mr. Misty
Jordy a.k.a. Bridger (when he is good)
Danson Flame - hey dad I'm well now and ready to go!

Brooksville, FL