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Calculating energy requirements for heavy runners

  1. Aug 5, 2009 #1
    Hi All...

    This is my first time, be gentle.

    It was a long time ago that I studied stage 1 physics during my biochemistry degree (yes, I know biochemistry was a cop-out) and I am having trouble figuring out a way to express the energy requirements for different body weight runners. Basically someone quoted F=MA to me to describe why a man weighing 90kgs running at a constant speed of 4m/s would require more force than a man weighing 60kgs also running at a constant speed of 4m/s. Now I know that it is obvious that the bigger bloke must exert more force but it seems that F=MA is not the formula to show this. What I really want to do is work out the relationship between body weight and force required to move that body weight at a constant speed.

    Any ideas?

    Thanks all,
    Andrew (from New Zealand)
  2. jcsd
  3. Aug 6, 2009 #2


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    Welcome to PF!

    Hi Andrew! Welcome to PF! :smile:
    Yes, at constant speed, the acceleration (A) is zero, so F = MA doesn't help at all!! :biggrin:

    At constant speed, the forces must balance, so the horizontal force from the muscles must equal the air resistance.

    So weight doesn't come into it, though frontal surface area certainly does. :smile:

    But I don't know how the horizontal force relates to the energy … it presumably depends on lots of physiology-type things. :redface:
  4. Aug 6, 2009 #3
    F = MA is indeed the short explanation, but not very useful in prediction. Every stride contains a vertical component that is an acceleration against gravity. Also muscles do not generate a constant force, but create a series of accelerations. You'll never get to an accurate math analysis without knowing and accounting for all the masses and motions of the individual body parts.
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