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Observing energy problem

  1. Oct 17, 2011 #1
    How can we observe an energy losing during a weight lifting?

    I mean when we lift a weight at different angle, using our hand, it would require different amount of energy.
    I repeat, energy, not force.
    How can we measure them in practical way?
  2. jcsd
  3. Oct 17, 2011 #2


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    This sounds like something that would come into sports medicine. They would probably do something sophisticated like measuring the lactic acid in the blood in that muscle, but I'm just guessing. Perhaps it would suffice for a rough comparison to repeat the exercise a dozen times and measure the increase in heart rate? Then rest for a while and repeat but this time using a more efficient exercise technique and you should note a reduced increase in pulse rate. For greater accuracy, later repeat same procedure but in reversed order, then average the results.
    Last edited: Oct 17, 2011
  4. Oct 17, 2011 #3


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    You could calculate the energy you apply to the weight. Since any work done by the body isn't fed back to the body, you can consider the energy loss to be at least equal to the energy you apply to the weight to lift it.
    This energy loss can be calculated in Joules, then converted to Calories.

    You also use energy when you do static work, for example lifting a 10kg weight and keeps it there by hand. You also use energy to move the weight down again.

    Just by keeping your arms straight out, you start to sweat even if no mechanical work has been done.

    Any static or dynamic force applied by the body require energy.

  5. Oct 17, 2011 #4


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    CO2 excretion rate would give a good idea of respiration rate for an activity which lasts for at least a few minutes. Just subtract the value when at rest. I've seen films with cyclists connected to gas analysis gear by large tubes.

    'Work Done' on the weight you are lifting would give a clue for a single lift. Weight (N) times height lifted (m) would give the Joules delivered. But, as Low-Q says, just keeping muscles under tension requires energy to be input. The individual fibres only hold the load for a few seconds and then go slack, transferring the load to others. The stretching and relaxing each time involves a constant amount of net Work (force times distance). I have no idea how you would use a 'mechanical' method to get any useful data out about Energy input.
  6. Oct 17, 2011 #5
    I have an idea that there must be at least a little work [itex]\epsilon[/itex] being done during this process(static).

    I assume that, in static system, to push something with a constant force and there was no work done on the object, our body has already lose its energy of [itex]\epsilon[/itex] Joules. This energy can be described as the work deterring the object from its moving. Also, [itex]\epsilon[/itex] is equal to the work deterring us from our moving(3rd Newton's law of motion).

    I have
    [itex]F = \mu\alpha = \mu\frac{d^{2}}{dt^{2}}\varsigma[/itex] ;
    for pretending mass(of arm) [itex]\mu[/itex]
    pretending acceleration(of arm) [itex]\alpha[/itex]
    and pretending distance(of arm) [itex]\varsigma[/itex]

    that is [itex]\varsigma = Ft^{2}/2\mu[/itex]

    So I have [itex]\epsilon = \int Fd\varsigma = (Ft)^{2}/2\mu[/itex]

    The quantity [itex]\mu[/itex] is what I'm interested.
    In the case of lifting object, I see that there must be some relation between the angle of arm and [itex]\mu[/itex]

    I don't know, but it must be like [itex]\mu = \mu_{0}/sinθ[/itex]

    that is [itex]\epsilon = (Ft)^{2}sin\theta/2\mu_{0}[/itex]

    I want to check my assumption with an experiment. Anyway, I decided not to play with ATP or respiration because I have no experience about them(which may cause an error). I'm interested in mechanical device, analogous with our arm. May be, there should be a device that could measure amount of potential energy of spring or some sort of this. but I have no idea.
  7. Oct 17, 2011 #6


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    Not according to physics. It just a property of muscles, that they "waste" energy when generating a (at the macroscopic level) static force.
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