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How much energy does it take to retain a running speed?

  1. Feb 1, 2013 #1
    My question revolves basically around how much energy would be required for a person to exert to remain at a constant speed. While exact numbers would obviously not be possible without a specific example (and would probably be impossible even with a specific example), I am thinking more relative figures.

    For example, for a 50 kg individual to move at 5 m/s, the Newtonian kinetic energy required would be 625 J (based on Ke=1/2mv2).

    I wouldn't think that the individual (ignoring air resistance) would essentially have to exert 625 J per step (as they are not starting from 0 each time), but I also would think that it would be quite a good chunk of the 625 J, as running 10 m is quite a bit less strenuous than running 100 m.

    I am a physics beginner who is simply interested, so I apologize if there are any fallacies in the scenario I have laid out. I appreciate your help!
  2. jcsd
  3. Feb 1, 2013 #2
    I think this requires more knowledge of biology than it does of physics.

    Note that an object in motion stays in motion and an object at rest stays at rest. This means that if you have an object moving at 5m/s with no air resistance or friction it requires zero energy to stay moving. An object in motion stays in motion... The energy you have calculated is the amount of kinetic energy an object has once its in motion. It is not how much energy is needed to maintain that motion.

    The human body is complicated. It requires energy just to sit there. If I wanted your answer I would look at sports medicine or kinesiology research or data.
  4. Feb 1, 2013 #3
    That's a really good point. My question, as I should have clarified, is more about what additional energy would need to be expended (from a physics standpoint, moreso than a biological one). So, how much more energy to propel an individual forward at 5 m/s as compared to an individual simply standing (this would eliminate the energy exerted as a form of the normal force).

    So the individual starts out still, converts 625 J of biological energy (I'm sure there is a more correct term for this) to kinetic energy and is moving 5 m/s. I guess, in my mind, the question revolves around the energy that is dissipated with each step. Yes, once in motion, the body would stay in motion. But each step is clearly requiring energy to maintain that level of motion.

    Once again, I realize this would not be something easily calculated, but I was just curious if anybody had a feel on it, as far as a relative range.

    Thanks again!
  5. Feb 1, 2013 #4
    Its a question of biology. I guess that experimental evidence would be superior to deriving it from a theory.

    Yes, your body would convert 625 J of biological energy into kinetic energy for your motion. It would also convert extra based on your physiology. How much extra? Depends... How efficient are your steps? How long is your stride? What is your heart rate and metabolism like? How hot is it outside? Lots and lots of potential factors. With so many factors its real hard to make a general formula that will give you an answer. I dont know anything about any of this, so I wont speculate too much. I suspect it varies quite a bit depending on if you are a fit athlete or a couch potato. I also suspect that in the kinesiology field there are models that have been developed to find this. Models that take into account the factors I mentioned (as well as other of course).
  6. Feb 1, 2013 #5
    When I was a jogger, I used the rule of thumb (based on what I had read) that I was expending an additional 100 kcal per mile.

  7. Feb 1, 2013 #6


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