Dear Stan,
Thanks for this article. I enjoyed reading it and I particularly
enjoyed your explanation of torque as I am not an engineer and the
diagram of the horse.
As I read your article, two thoughts occurred to me.
1 I recall my first physics teacher telling me that Watt lifted a
weight vertically in a well using the windlass on the well to
translate the vertical force of the weight to the dragline attached to
the horse's harness. Either way, it would produce the same effect but
with one less pulley.
2 The same physics teacher told us that James Watt also cheated when
calculating the 'power rating' of his horses by understating the size
of a horsepower. The story was that Watt chose the largest strongest
(most powerful) horses that he could find and used these for his
tests. He then rounded down their performance from (say) 590 to 550
foot pound per second. The idea behind this subterfuge was that he was
sure that after he had sold and installed a one horsepower engine he
would be barely out the gate before the new owner would try out his
new steam engine against a real horse. I am sure that subterfuges such
as this were quite common prior to the international standardisation
made possible by the introduction of the International System of Units
(SI). Watt might have been unique in the creation of the horsepower in
that he did it for an relatively honest reason rather than for
cheating and/or obfuscation.
Cheers,
Pat Naughtin
Geelong, Australia
On 2008/12/12, at 1:13 AM, Stan Jakuba wrote:
I am responding to Pat's challenge to help with the explanation of
power, that is power measured in W (not in votes, for example, as
might apply to political power). There are numerous attachments in
these e-mails, arranged to facilitate their opening.
The first group is a scanned article that illustrates the meaning of
mechanical power and of work (as a form of energy). It also covers
torque (it is in the title) thereby touching on the other topic
frequent on this forum - why N·m is a unit of torque (couple) as is,
but N·m as work is called J.
The second one compares power and energy in nutrition and in
workouts. It has been presented on this forum many times. To obtain
the feel for W and J, read the last footnotes.
The next one is a table of food energy (heat) intake and power
output in bicycling and in walking. In the footnotes it covers some
terminology issues, and shows that men's metabolic-to-output
efficiency is on par with any comparably-sized combustion engine,
zoological or mechanical. The table has also been presented here
before.
The last one is a table similar to the preceding one. It presents
several physical activities and shows when power in W equals
numerically energy in kJ. It had also been here before.
While at this, I just thought that you might want to check you
knowledge. Thus the last attachment. I will be glad to check
anyone's answers and provide corrections, if needed. Refer to the
question numbers.
Stan Jakuba
----- Original Message -----
From: Pat Naughtin
To: U.S. Metric Association
Cc: U.S. Metric Association
Sent: 08 Dec 03, Wednesday 00:19
Subject: [USMA:42118] Re: Problems with power
Dear Bill and Jesse,
First, thank you Bill for responding to Jesse's question. I have to
admit that I ducked when he first proposed it as I have always found
it difficult to explain the difference between energy and power. I
was hoping that someone more knowledgeable about physics would jump
in. I particularly like Bill's line: power is instantaneous; energy
requires time (even if that time is only a picosecond) as better
than the usual dull definitions:
Energy is the ability to do work.
Power is the time rate of doing work or of using energy.
By the way, as I was searching for a better answer to Jesse's
question, I came across two interesting web sites. The first is the
USA Government Web at http://www.usa.gov/Citizen/Topics/Environment_Agriculture/Energy.shtml
where I found tutorials such as this http://www.glenbrook.k12.il.us/gbssci/phys/Class/energy/u5l1a.html
but I was not happy that these explained in suitable terms an
answer to Jesse's initial question. However, I soon became confused
by the diversity of measuring words used on this web site. As you
know I have found some 199 different words to describe energy
(requiring 39 402 conversion factors) with old pre-metric measures,
when only 1 unit is needed for the metric system (with no conversion
factors at all); see http://www.metricationmatters.com/docs/EnergyWords.pdf
On the way, I found this interesting comparison of energy sources http://physics.syr.edu/courses/modules/ENERGY/ENERGY_POLICY/tables.html
although I was disappointed in the muddled old pre-metric measures
placed in with the correct use of SI units.
Cheers,
Pat Naughtin
P.S. I shall mull some more!
On 2008/12/03, at 3:16 PM, Bill Potts wrote:
I agree with you Jesse.
5 W represents power and 5 J represents the energy used by a 5 W
device in 1
s.
Put another way, power is instantaneous; energy requires time (even
if that
time is only a picosecond).
Bill
________________________________
Bill Potts
WFP Consulting
Roseville, CA
http://metric1.org [SI Navigator]
-----Original Message-----
From: [email protected] [mailto:[email protected]] On
Behalf
Of Ziser, Jesse
Sent: Tuesday, December 02, 2008 17:45
To: U.S. Metric Association
Subject: [USMA:42115] Re: Problems with power
--- Pat Naughtin <[email protected]> wrote:
supply the power needed - supply the energy needed (Again a
difficult
idea as the engineers have to provide an electrical plant with a
power
rating that will provide the maximum amount of energy needed at a
particular time)
This in particular illustrates the problem I have with your use of
the term.
It sounds like
you're saying that a phrase like "supply the 5 watts of power
needed by the
motor" is incorrect
and it should be "supply the 5 watts of energy needed by the
motor". Is
that what you're saying?
Because this conflicts with the definition of the term "power" as I
learned
it in school.
And a power rating specifying the amount of energy needed at one
time? Well
that's just gibberish
to me.
Please explain in more detail why you think these uses of "power"
should be
changed. I am
confused.
Pat Naughtin
PO Box 305 Belmont 3216,
Geelong, Australia
Phone: 61 3 5241 2008
Metric system consultant, writer, and speaker, Pat Naughtin, has
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