Re: [Phys-l] Food Liar's calorie chart

[Philip Keller <PKeller@holmdelschools.org>]

This reminds me of a question I've had for a while.  I have not found the answers despite extensive research (i.e. googled once or twice).

Does the Carnot efficiency provide an upper limit on the efficiency of the human engine?  If so, does that mean that my efficiency at turning food calories into mechanical energy is limited by the temperatures...and what temperatures would those be?  Body and ambient?  If so, that explains why the vast majority of the food calories goes to emitting heat!
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From: phys-l-bounces@carnot.physics.buffalo.edu [phys-l-bounces@carnot.physics.buffalo.edu] On Behalf Of curtis osterhoudt [flutzpah@yahoo.com]
Sent: Friday, April 17, 2009 3:13 PM
To: Forum for Physics Educators
Subject: Re: [Phys-l] Food Liar's calorie chart

Watching Walter Lewin's lecture on how much energy people actually use during exercise is fascinating. (It's lecture 14 in his classical mechanics lectures from MIT). The gist of it is that the vast majority of energy use is used for basic things, like keeping the body emitting ~100 W all the time.

Only if one does major expeditions, or runs marathons or the like will that be a significant addition to the normal basal rate. Try http://ocw.mit.edu/OcwWeb/Physics/8-01Physics-IFall1999/VideoLectures/detail/embed14.htm for some possible videos, and a transcript.



 /************************************
Down with categorical imperative!
flutzpah@yahoo.com
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From: Stefan Jeglinski <jeglin@4pi.com>
To: Forum for Physics Educators <phys-l@carnot.physics.buffalo.edu>
Sent: Friday, April 17, 2009 12:59:11 PM
Subject: Re: [Phys-l] Food Liar's calorie chart

> Here, the dominant effect is the rhythmic contraction of various muscles
> which augment the energy converted to heat and to pumping blood to the
> brain.

Based on 2400 nutritional calories per day, we get 50 cal in 30 min
as a baseline metabolism, which includes baseline rhythmic
contractions, heat dissipation, and blood pumping. The chart I refer
to is not sophisticated enough to describe whether the calorie counts
are in addition to or part of the metabolism, but the numbers to me
imply in addition to (if the chart had an entry for "no activity" it
would read 0 cal). My point remains - the difference between putting
yourself in motion at 4mph and 5mph is not much. Let's say the
metabolic elevation during exercise doubles to 100, and add that to
the chart levels:

370 + 100 = 470 (jogging 5 mph)
200 + 100 = 300 (walking 4 mph)

It's a large discrepancy no matter how you cut it.

I've not been able to go through the plethora of reference materials,
but I have found at least one that claims:

F = 140 cal/mi (jogging, 200lb person) (vs my quote of 148)
F = 133 cal/mi (walking, 200lb person) (vs my quote of 100)

This is much more in line with my original thesis that the numbers in
the chart I cited are suspicious. I *might* believe that the
metabolic difference and possible** changes in CM make the difference
between 140 and 133, but not the other.

Bottom line point - variability in metabolism and nuances of the
exact form of exercise (the details of F.dx) make small differences
not worth arguing about. Determining inconsistencies in calorie
charts handed down like bibles, and explaining them (or being unable
to), make for a good exercise. I'll leave this part of the discussion
at that :-)


>  >  it is F.x that determines the amount of energy expended.
>  >
> Life-forms don't quite follow this relation, which works rather well for
> all too solid masses of other kinds.

Life-forms can't escape the physics that works for solid masses of
other kinds, I contend. Aside from thermodynamic considerations
included in the base metabolism, F.x (F.dx more appropriately)
determines energy expenditure in pumping blood, rhythmic contraction
of muscles, and moving from point A to point B on a walking/jogging
surface.

As to treadmills and Bob at PC's humorous attempt to strike fear in
anorexics everywhere - when running or walking, I claim to be on a
treadmill called earth. True, I can't really see it moving backwards
under my feet... or can I? The student can also ponder the energy
differences, if any, of running/walking on a stationary train, or a
train that is moving in the same direction, or opposite...

Real treadmills (like in the gym) would have some differing physics
as to energy dissipation (just ask any walker runner and they'll tell
you it just ain't quite the same as "real" walking or jogging).
Treadmills are much better at differentiating "secondary energy
dissipation" vs running outdoors (wildly swinging limbs, subtle
terrain variations, wind resistance, psychological factors, etc etc).

**With respect to CM, a few minutes watching Monty Python's silly
walks skit might convince that you can train yourself to walk or run
with very little change in vertical CM. For a more convincing
demonstration, watch someone skilled at moving in Tai Chi or another
martial art. I'm not buying the change in CM argument (running vs
walking) at this point. Gotta work the numbers, but I'm not confident
there's a big difference.


Stefan Jeglinski
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_______________________________________________
Forum for Physics Educators
Phys-l@carnot.physics.buffalo.edu
https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l