Warp wrote:
> How does GR affect this? Or does it affect at all?

I believe that at least for this case, the energy change shows up as a 
change in "m".  In other words, "F=ma" still holds, but you use E=mc^2 to 
calculate the actual mass.  (At least in special relativity, that's how you 
work it.)



-- 
Darren New, San Diego CA, USA (PST)
    C# - a language whose greatest drawback
    is that its best implementation comes
    from a company that doesn't hate Microsoft.

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Orchid XP v8 wrote:
> According to the textbooks, it takes 1 Jule to move a 1 kg object a 
> distance of 1 meter.

If one Newton force is applied in the direction of motion for the whole 
distance, yes.

> On the other hand, once the object has been moved, keeping it stationary 
> requires no energy at all. And, indeed, if you take a lump of metal and 
> put it on your bookshelf, it requires no energy to make it remain there. 
> It just sits there like a lifeless lump of metal.

Quite true.

> Now, here's the thing: How much energy does it take to hold a 1 kg lump 
> of metal at arm's legnth?
> 
> According to physics, it would require 0 Jules.

If this were true, then it would take no effort to do this.  You concept 
of this being "according to physics" is off a bit.


> However, to keep the 
> object stationary against the force of gravity, the muscles in your arm 
> are having to continually expend chemical energy.

This is because the 1kg object is resting on a system of tensed muscles, 
instead of a rigid framework.  The tense muscles expend energy in order 
to maintain their tension.

> But how the **** do you compute how much energy that is??

Exercise physiologists would probably measure the oxygen consumption of 
a person at rest vs. a person holding the weight at arm's length.  The 
difference corresponds to the extra requirements.

Regards,
John

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Am 31.07.2010 16:21, schrieb Orchid XP v8:

> According to physics, it would require 0 Jules. However, to keep the
> object stationary against the force of gravity, the muscles in your arm
> are having to continually expend chemical energy. But how the **** do
> you compute how much energy that is??

Measure the amount of Co2 you exhale (a) normally, and (b) while 
performing said feat, then from the difference compute the extra Co2 
produced for that feat, and from that estimate the energy?

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Am 31.07.2010 16:38, schrieb Stephen:

>> According to physics, it would require 0 Jules. However, to keep the
>> object stationary against the force of gravity, the muscles in your arm
>> are having to continually expend chemical energy. But how the **** do
>> you compute how much energy that is??
>
> Analyse the system and come back with an answer. This is 3rd year high
> school mechanics.

... plus university level biochemistry, yes.

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Am 31.07.2010 22:36, schrieb Orchid XP v8:

>> 1 Joule is also the energy transformed in heat in 1 second by a 1 Ohm
>> resistor when the current is 1 ampere.
>
> And here I was thinking that the heat produced depends on the
> characteristics of the material, not just the current...

In a sense that's true - but the material characteristics influence the 
resistance /exactly/ the same way as the heat produced ;-)

> I guess ultimately, muscles generate forces. Presumably to hold an
> object still, the force generated by the total muscle system must be
> equal (and opposite to) the force of gravity. It's all quite
> complicated, since most forms of locomation involve levering, and you'd
> have to know muscle insertion points and pivot lengths and so on.

The thing with muscles is, they never actually hold something still 
unless they're totally slack. When they're contracted, they actually 
twitch at a high frequency, thus repetitively lifting the weight up a 
bit, converting chemical energy to potential energy, then letting the 
weight drop a bit only to catch it again a split second later, wasting 
some of the energy as heat. (The tendons help "recycling" some of the 
kinetic energy by converting it into mechanical energy like a spring, 
and kangaroos make pretty effective use of this mechanism for moving 
around, but even they lose part of the energy as heat, thus effecting 
global warming and ultimately the entropy death of the universe... but 
maybe I'm drifting off here :-).)

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>> According to physics, it would require 0 Jules. However, to keep the
>> object stationary against the force of gravity, the muscles in your arm
>> are having to continually expend chemical energy. But how the **** do
>> you compute how much energy that is??
> 
> Measure the amount of Co2 you exhale (a) normally, and (b) while 
> performing said feat, then from the difference compute the extra Co2 
> produced for that feat, and from that estimate the energy?

Sure. Now tell me how much energy is released by the production of 1 
unit of CO2...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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>>> 1 Joule is also the energy transformed in heat in 1 second by a 1 Ohm
>>> resistor when the current is 1 ampere.
>>
>> And here I was thinking that the heat produced depends on the
>> characteristics of the material, not just the current...
> 
> In a sense that's true - but the material characteristics influence the 
> resistance /exactly/ the same way as the heat produced ;-)

Hmm, interesting.

> The thing with muscles is, they never actually hold something still 
> unless they're totally slack. When they're contracted, they actually 
> twitch at a high frequency

Individual fibers twitch, yes. A single "muscle" contains rather a lot 
of individual fibers though, all twitching in a coordinated pattern. The 
net result is more or less contant tension.

I'm told this is why when you try to lift something too heavy, you start 
shaking. You run out of fibers to twitch.

(Related... I'm told that if *all* the fibers were to twitch at once, 
the muscle would probably tear itself from its attachment points. Which 
perhaps explains why people undergoing convulsions sometimes display 
superhuman strength.)

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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Am 01.08.2010 18:16, schrieb Orchid XP v8:

>> The thing with muscles is, they never actually hold something still
>> unless they're totally slack. When they're contracted, they actually
>> twitch at a high frequency
>
> Individual fibers twitch, yes. A single "muscle" contains rather a lot
> of individual fibers though, all twitching in a coordinated pattern. The
> net result is more or less contant tension.
>
> I'm told this is why when you try to lift something too heavy, you start
> shaking. You run out of fibers to twitch.
>
> (Related... I'm told that if *all* the fibers were to twitch at once,
> the muscle would probably tear itself from its attachment points. Which
> perhaps explains why people undergoing convulsions sometimes display
> superhuman strength.)

Hm, I didn't know /that/ (and I'm still doubtful about it). Still, the 
result is only "more or less constant" tension, so there /is/ some 
inevitable net twitching.

If it's true though, then the energy you burn to hold something steady 
is actually the energy you need to overcome the friction in your muscles.

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Am 01.08.2010 18:12, schrieb Orchid XP v8:
>>> According to physics, it would require 0 Jules. However, to keep the
>>> object stationary against the force of gravity, the muscles in your arm
>>> are having to continually expend chemical energy. But how the **** do
>>> you compute how much energy that is??
>>
>> Measure the amount of Co2 you exhale (a) normally, and (b) while
>> performing said feat, then from the difference compute the extra Co2
>> produced for that feat, and from that estimate the energy?
>
> Sure. Now tell me how much energy is released by the production of 1
> unit of CO2...

I guess some biochemistry experts have already done elaborate studies on 
this subject, so it's just a matter of finding the right paper to give 
you this answer.

For instance, you could do the same measurements on a person working out 
on an ergometer (a high-quality "calibrated" treadmill, stationary 
bicycle or similar device) at different loads.

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clipka wrote:
> I guess some biochemistry experts have already done elaborate studies on 
> this subject, so it's just a matter of finding the right paper to give 
> you this answer.

Oooo! Ooooo!  I know that one!


http://en.wikipedia.org/wiki/Citric_acid_cycle



-- 
Darren New, San Diego CA, USA (PST)
    C# - a language whose greatest drawback
    is that its best implementation comes
    from a company that doesn't hate Microsoft.

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