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



That URL from Curtis, of Lewin's talk, mentioned a food conversion efficiency of 20%
But Lewin had in mind stoking that 100 watt built in heater that we tote, lying, sitting or walking.

That number is very convenient for the memory:
100 watt for 60 X 60 X 24 seconds per day amounts to 8.64 MJ pd or in the customary unit - 2.06 Mcal or in the even more familiar classical unit, 2060 Calories. Somewhat lean cuisine for the average generously-sized American shape, I suppose.

The underlying conversion can be surprizingly efficient, I seem to recall.
It is not limited to the Carnot criterion because the metabolic conversions are not a heat engine effect - rather more like a fuel cell (can be 80%??)

Brian W

Philip Keller wrote:
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!
________________________________________
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.