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Applied Physics: Weight-Lifting Work

  1. May 14, 2007 #1
    Applied Physics: Weight-Lifting "Work"

    OK, sorry, this may be in the wrong place.... but where would you post something like this?

    This may be too simple, but I cannot get my mind wrapped around this. I need to lose weight and I want to be able to 'quantify' the calories I burn. If I am bench pressing, and I raise 100 lbs. through a distance of 24 inches, how much 'work' have I done, in terms of calories burned? We can neglect all the variables such as efficiency, speed of movement, work done in 'lowering' the weight, etc. I just want to know what the minimum work (change in potential energy) is, in terms of calories required.

    My goal is to be able to say, "I want to burn X calories per day. How much weight do I have to move per day (through the 24" distance) to burn at least that many calories?"

    Thanks in advance for your help.
    Last edited: May 14, 2007
  2. jcsd
  3. May 14, 2007 #2


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    Science Advisor

    In the basic sense, you can calculate the work and then convert over to calories. Just remember that a food calorie is actually 1 kcal in engineering units (4317 J). You do realize though that this is an extremely inaccurate way to do this calculation...
  4. May 14, 2007 #3


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    Staff: Mentor

    Just to amplify - the reason this is in accurate is because in a physics sense, you should be absorbing work when you bring the weights back down, but you still actually expend energy.
  5. May 14, 2007 #4
    russ, I think that would be true if you aren't fighting gravity.

    For example, I push weights up, gravity does negative work and I do positive work. I let the weights fall, I do no work, gravity does positive work.

    However, if I SLOWLY bring the weights down, while gravity wants to bring the weights down faster, I apply a force (not great enough to accelerate the weights up) bringing the weights down slowly. So gravity does positive work, and I do negative work. But all the same, I'm STILL doing work.
  6. May 14, 2007 #5
    I lift, I have thought about this before. Unfortunately physics alone can't supply the answer without biology.

    If we just consider the work done against gravity it would take (one rep of) a 200 pound bench press to burn one human food calorie.

    From here the issue comes down to efficiency, how much effort is wasted? (A human can expend calories in muscle tension without doing any work, saying nothing of our core bio functions that run all the time).

    Here is what you need to do: use a cardio machine that will measure your heart rate, and tell you the wattage of your effort.

    Once you know that heart rate x leads to wattage y, just multiply the y of your average heartrate by the length of your workout (measured in seconds)
    and by 1/4317 to get a vague idea of your expended calories.

    If you want very detailed data of this kind specific to your own body, you would need to spend a few hour taking data and creating look-up tables. I wonder if I should start offering this as a proffesional service to people at my local gym and over the internet.
  7. May 14, 2007 #6


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    Staff: Mentor

    I know - I think you may have misread my post.

    Btw, when lifting weights, you do lower the weights slowly for precisely that reason.
  8. May 15, 2007 #7

    Doc Al

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    Staff: Mentor

    Careful here. When lifting the weight, you do positive work against gravity: you add mechanical energy to the system (barbell + earth). When lowering the weight slowly, gravity is doing positive work. The system (barbell + earth) loses mechanical energy, which you gain. You gain the energy that the barbell loses! Of course, that "lost" energy gets transformed into an increase in your internal energy--your muscles heat up.

    (Just saying you "do work", albeit negative, is a bit misleading. It's like saying, after someone gives you $10, that you gave them negative $10. Either way you are the one getting the $10.)

    The real problem, as Crosson points out, is that we are talking about a biological system here, not just a barbell. It requires chemical energy (fuel = calories) to just hold the barbell in position, never mind lift it up or down. If you examine the muscle fibers in action, they are continually contracting and relaxing, doing positive work just to maintain overall tension. You expend chemical energy (fuel) both when lifting and lowering the weight.
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