Re: Mechanical, chemical and heat energy

[Susan Evans Garlinghouse <suendavid@worldnet.att.net>]

Truman Prevatt wrote:
> 
> 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.

Sure.  But you know and I know that there are few assumptions you can
make in the science community without having to back it up with a
validation study and thus a reference.  And if they HADN'T come to that
conclusion, well, it wouldn't be the first time we all made an
assumption and found out it really DIDN'T work out that way.  Horses are
weird creatures and just not very predictable.  And the point of the
study was to develop an equation for energy consumption---the fact that
they validated equal energetics between equal weight loads was just a
"oh, by the way" sort of thing.
> 
> 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. 

They used riders, not weight saddles.  I don't have the study right here
in front of me, but they used the same experienced rider(s) on all the
trial horses.  I don't think they had more than a few riders in total. 
How that directly affected the horse's center of mass, I don't know.





> 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'm not so sure it's as simple as that, Truman.  This is getting out of
my area, but my understanding is that it's not just surface area, but a
ratio between body mass and surface area.  So as body mass increases,
the surface area per kg of body mass is decreasing, thus making it more
difficult to dissipate heat as the body size increases.  This is why
animals that live in extremely hot zones tend to be smaller with
adaptations to increase surface area further (like mice or a bat-eared
fox) and animals in extremely cold habitats tend to be larger and
bulkier (like a blue whale in Anarctic waters) with adaptations to
further decrease surface area (shorter extremities with limited blood
flow).  There are always exceptions, but animals will always evolve
adaptations in surface area (upwards or downwards) to handle their
environmental conditions.


 
> 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.

Ahh, but therein lies the rub.  Many researchers are interested in
research for pure research's sake, not whether or not the horse
community ever gets anything practical out of it.  I happen to be an
applied scientist, but alot of researchers aren't, for various reasons
that are perfectly valid.  Wasn't your field of study mathematics,
Truman?  Was your research simply in developing or validating equations
for equation's sake, or did it ever have a practical application, like
making it easier to balance a checkbook?  Same things apply in the
biological sciences---not every study makes it to the lay community, nor
should it.

Also, under field or laboratory conditions, heat production in
exercising horse's (or even a sedentary horse) is extremely difficult,
if not impossible, to measure---it takes a metabolism stall and that's
hard even with little critters, let alone a big animal like a horse.  To
get an accurate picture of heat production, you have to directly measure
the rise in air temperature as a result of metabolism, the energy
contained in sweat production, and the energy contained in expired gases
(including methane), urine, and feces.  One of my advisors built a
metabolism stall the right size for sheep and I think it cost about
$100,000.  I'm not sure, but I think there are only two or three
metabolism stalls in the country big enough for a horse, and none of
those can measure heat production during exercise.

And so we muddle along as best we can, but we never claim that one study
can or will explain every last factor that may or may not be
applicable.  This particular study of Pagan & Hint'z just based energy
expenditure on oxygen consumption, which is actually pretty accurate,
but heat production wasn't part of the study, nor could it have been.

Susan G