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Old 05-28-2009, 03:22 PM   #1
Jonathan Silverman
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Physics question

In physics, what is the relationship between power and efficiency?
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Old 05-28-2009, 03:24 PM   #2
Jamie J. Skibicki
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Re: Physics question

Power and efficiency? I'm not sure what you mean
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Old 05-28-2009, 03:26 PM   #3
Dan Morgan
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Re: Physics question

Efficiency is the amount of useful power output of a system as a percentage of the total power input.
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Old 05-28-2009, 03:27 PM   #4
Jonathan Silverman
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Re: Physics question

well the question comes from this:

crossfit talks about high power ouputs and that's what strong is.

and Rip connects proper technique, efficient use of geometry in the work of lifting as what strong is.

So i was wondering what the relationship is between energy efficiency and power is...
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Old 05-28-2009, 03:36 PM   #5
Dan Morgan
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Re: Physics question

In relation to the deadlift, for example. The total amount of work that needs to be done, or the 'useful power output' is proportional to the height that the weight needs to be lifted off the floor, it is a constant value.

If an athlete lifts the bar around their knees, ie. not in a vertical line from the floor they have done unneccessary work, and thus a percentage of their total power output has not been 'useful power output', and so they are less efficient.

I hope that makes sense.
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Old 05-28-2009, 03:42 PM   #6
James E Phillips
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Re: Physics question

I think a more useful relationship to discuss would be that between energy and efficiency...

Your energy output becomes more efficient when more of the energy contributes to your intended goal.

For example, the goal in oly lifting is lifting the weight to an overhead position. The better your form, the more efficient you will be. Poor form would be wasteful in that energy is used to move the weight in a direction that is counterproductive, ie. not overhead.

Sometimes, you want something to be inefficient such as resistors in a circuit.

You want to manipulate the electricity flowing in a wire, so you use a resistor to decrease voltage and/or current. That energy has to go somewhere, so the resistance dissipates it as heat. The same is true with friction - friction creates heat and sound. A door hinge creaking means indicates that there's a significant amount of friction in the hinge and adding some WD-40 will make the door more "efficient" ie, it will take less effort to accelerate the mass of the door because energy will not be wasted making sound...

As far as power is concerned...what is power? Energy expended over a certain amount of time.

Was that anywhere near what you were asking?
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Old 05-28-2009, 03:43 PM   #7
Jonathan Silverman
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Re: Physics question

yeah. Thanks.
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Old 05-28-2009, 03:51 PM   #8
Jamie J. Skibicki
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Re: Physics question

WHat do you mean by energy efficency?

The more mechanical energy you create in the same amount of time, the more power you get. The less time it takes you to create the same amount of energy, the more power you create.

But for the love of god, stop trying to apply basic mechanics to the human body indiscriminately, it doesn't work for a few reasons.

First reason, there is more to body than mechanical work, mainly chemical work. Holding an iron cross takes Zero work which mean zero power. But it takes a HUGE amount of force generation since our bodies are not set up for static holds. But using the "increasing power over broad time domains" doesn't work here.

Second reason, there is the setup of the body and the relative strength of the muscle involved. A bench press and an OH press have pretty much the same ROM, therefore, the same change in energy from the bottom position to the top position. But, you cannot (unless you really trained for it or are built very weird) produce as much mechanical work by doing OH press as compared to the bench since you pecs are stronger than you delts and there relatively good levers.

SO the short answer is, there isn't any relation between efficiency and power.

Crossfit says power generation is what is fit.

What rip is talking about is maximizing force generation given the constraints of the human body.

What is efficient changes with the particulars of the person involved and the length of the event.

If you have a short event (mainly something falling within ATP energy cycle) efficient is what allows you to create as much force as possible, since fatigued is not an issue here.

If you have long event, efficient is what allows you to keep going at a good pace. You might sacrifice mechanical efficiency (The ratio of chemical energy to mechanical energy) to reduce the stress of a particular muscle group so it doesn't prematurely fail.

Wait maybe thats what you mean, what the relationship between chemical energy conversion to produced mechanical work?
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Old 05-28-2009, 03:59 PM   #9
Jonathan Silverman
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Re: Physics question

Quote:
Originally Posted by Jamie J. Skibicki;

Wait maybe thats what you mean, what the relationship between chemical energy conversion to produced mechanical work?
I don't know what I mean anymore. But that's a gr8 question and if you wanna roll with it be my guest.

One prolly needs a biochemist at this point.
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Old 05-28-2009, 04:21 PM   #10
Tom Woodward
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Re: Physics question

Anyone intrigued by Jamie's post should check out this thread:

http://************.net/forum/showthread.php?t=5032 (not WFS due to language)

As Jamie says, isometric work generates zero work but requires energy consumption. This jives with Rip saying that more ATP is consumed while holding the erector spinae in ideal positioning than not, since that correct position for the lumbar is isometric. It logically follows that more mechanical work can be done for a longer period of time while guzzling less ATP by NOT maintaining ideal isometric contraction of the lower back in an exercise like high rep deadlifts. Interesting stuff....

Last edited by Tom Woodward : 05-28-2009 at 04:23 PM.
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