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Math Help - Energy consumption when lifting weights

  1. #1
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    Energy consumption when lifting weights

    Hello.

    This is probably quite basic, but it's been a bit more than 20 years since I studied math/physics, so that's why I turn to you guys. I am trying to figure out how to calculate the energy consumption during weight lifting.

    I guess there's five calculations;
    1. Lifting a weight in a straight line
    2. Lowering a weight in a straight line
    3. Lifting a weight with a circular motion
    4. Lowering a weight with a circular motion
    5. Holding a weight in a fixed position


    I think I got the first one right.

    Using these definitions,
    • Mass (m) = 90 kg
    • Distance (h) = 0.48 m
    • Gravity multiplier (g) = 9.82
    • Joule (J) to calorie (cal) multiplier = 4.2

    I get this;

    J = mgh = 90 x 0.48 x 9.82 = 424.224
    cal = J/4.2 = 424.224/4.2 \approx 101

    (However, I am uncertain of this as it sounds quite a lot to use 101 calories to lift 90 kg 0.48 m, compared to, say, about 500 calories by running for 20 minutes.)


    For the second item in the list, I guess time needs to be added as a variable, as it's "slowing down the g-speed" (if dropping, the item would fall by 9.82 m/s2, right?).

    Number three and four will probably be some variants of one and two and I'm not even sure if they're relevant, but I suspect they may be, since the lifting/lowering don't happen along a straight line.

    The last one, number five, should, imho, be the same as number one, but since the weight ain't moving neither up or down, the distance will be 0 and as such the calculation fails. This tells me I don't understand this one at all.



    I'm sorry for turning to you with such (probably simple) problems and with such limited pre-knowledge, but in my attempts to find any formulas for this on the web, I've failed for about six months now. If anyone could help me out a bit, I'd be a very happy panda.
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  2. #2
    Grand Panjandrum
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    Re: Energy consumption when lifting weights

    Quote Originally Posted by PatrikBirgersson View Post
    Hello.

    This is probably quite basic, but it's been a bit more than 20 years since I studied math/physics, so that's why I turn to you guys. I am trying to figure out how to calculate the energy consumption during weight lifting.

    I guess there's five calculations;
    1. Lifting a weight in a straight line
    2. Lowering a weight in a straight line
    3. Lifting a weight with a circular motion
    4. Lowering a weight with a circular motion
    5. Holding a weight in a fixed position


    I think I got the first one right.

    Using these definitions,
    • Mass (m) = 90 kg
    • Distance (h) = 0.48 m
    • Gravity multiplier (g) = 9.82
    • Joule (J) to calorie (cal) multiplier = 4.2

    I get this;

    J = mgh = 90 x 0.48 x 9.82 = 424.224
    cal = J/4.2 = 424.224/4.2 \approx 101

    (However, I am uncertain of this as it sounds quite a lot to use 101 calories to lift 90 kg 0.48 m, compared to, say, about 500 calories by running for 20 minutes.)
    Food "calories" are kilo-calories.

    CB
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  3. #3
    MHF Contributor

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    Re: Energy consumption when lifting weights

    The "work done" will be "mgh" but your muscles and joints are not perfectly "efficient". Even holding a weight a "constant height", your muscles "slip" a little and then have to move the weight back up.
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