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Work vs. work (or physics vs. biology)

  1. Feb 3, 2010 #1
    Hopefully with my limited knowledge of physics I can make this make sense, and please correct me if I'm mis-using any terms.

    What I am trying to figure out at least a rudimentary model or explanation for is how "work" peformed by the human body varies based on efficiency for the same "work".

    For my first example, simply standing in place, you are not doing work by the physics definition because there is no motion. HOWEVER, obviously the muscles are doing work (using energy) to stand there. Bend your knees to about 90 degrees, and then stand there--again no "work" by the physics definition because you're not moving, but your muscles are at a much higher level of exertion and "working" awfully hard to keep you there. So with no "work" being done external to the body, there is a huge difference in work being done by the muscles. I guess let me back up and ask this--is quantifying muscular "work" essentially reverse engineered by energy usage?

    So where I'm ultimately headed is that lifing 100lb barbell 1 meter off the floor is a fixed workload from a physics standpoint. But from a biological standpoint, accomplishing this work can cover a spectrum of muscular (not to mention cardiovascular) "work" based on various body angles and muscle recruitment.

    Any fatal flaws here? Anyone got easy way to speak to the difference between the external work accomplished (lifting the weight) vs. internal work (energy expendature)?
     
  2. jcsd
  3. Feb 3, 2010 #2
    I'm really not sure on the cellular level what causes a muscle to burn energy despite not doing any "work", but you should note that this not only a biological phenomenon. A helicopter hovering in air, for example, doesn't perform any "work", and yet burns alot of fuel to keep aloft. You can floor the gas and the brake simultaneously on a car with an auto transmission, and no "work" will be done, despite burning alot of fuel and making alot of noise.

    Truthfully, what happens is that all these processes DO perform some type of work, just not necessarily what you want. A hovering helicopter moves the air around it, and creates alot of heat and noise. The torque converter in the auto transmission does work on the fluid inside it, ultimately to be wasted as heat. A musle fiber, likewise, probably simply wastes most of its effort on heat and movements on the cellular level, which don't necessarily translate into any visible work.
     
  4. Feb 3, 2010 #3
    While holding the object at a fixed height, you are not doing any work on that object. I.e. there is a force from your hands that acts on the object, but this force is not doing any work since the object is not moving. However, the human body is doing a lot of work within itself in order to produce the lifting force, since it is quite inefficient. All the different microscopic electrical/chemical processes going on in there are doing work, and the result is that you get exhausted if the weight is large.

    Torquil
     
  5. Feb 3, 2010 #4
    So maybe my question is actually much simpler and should have been how do you measure work done by the body--is it just calories burned??
     
  6. Feb 3, 2010 #5
    Calories is a measure of work.

    1 Calorie = 4186.8 joule. So...yeah. (Make sure to capitalize the [C]alorie)
     
  7. Feb 3, 2010 #6

    turin

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    Your confusion can be resolved, and you can also resolve these two notions of "work", by simply appealing to the First Law of Thermo:

    ΔU=Q-W

    where U is the energy of the object (i.e. person), Q is the heat transferred into the object (and is negative if the person is generating heat), and W is the work done by the object. (Note that the sign conventions may be different in different references.)

    What you refer to as physics work is the W. What you refer to as biological work is probably U (or more precisely ΔU).

    This is actually intimately related to the notion of "burning Calories". In fact, a Calorie is a unit of heat, that is, a unit to measure Q, not W. (Of course, Q and W have the same fundamental unit: energy.)

    Your body burns Calories just by being alive (the cells are constantly doing something, e.g. synthesizing protein, replicating, etc., not to mention electrochemical energy used in the nervous system and heart, persistently expanding and contracting diaphram for the lungs, etc.). Then, there is the issue of muscle tone. Your muscles behave dynamically. Even if the muscle is in a contracted state, the individual cells alternate between relaxed and contracted states. If you have good muscle tone, they remain contracted more. If you have poor muscle tone, the fibers are normally relaxed, and so they must be repeatedly contracted in order to sustain an extra tension. This contractions consumes chemical energy, and reduces the U in your body. The result is a negative Q.

    In other words, work is only one of the two basic forms of energy transfer.
     
  8. Feb 3, 2010 #7
    I read somewhere that the body can convert Big Calories to work (F dx) at about 10% efficiency.

    For the human body, 2000 Calories per day ≡ 97 watts (definition) = 97 joules per second. Any excess Calories x 10% may represent real joule work (e.g., lumberjacks).

    Bob S
     
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