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Mechanical Energy and Electricity

  1. Sep 25, 2008 #1
    Hi, my question is rather simple.
    Supose you can generate 10 000N, by any means, such as pedaling a bycicle.
    What calculations should you make to know how much electricity would 10 000 N of force will generate.
    -Assuming a generator efficiency of 100%, leaving friction aside, etc.
    An example would be, how much electricity will i produce by pulling a string attached to the rotor of the generator, and the total force that will exert during 1 hour will be 10,000 N?
    If you also please tell me how i can convert the result to kWh?

    I have a very basic understanding of physics, and if you think i would not be able to understand your answer at this stage, may you please tell me what prior reading i should make before asking this question, or if you may give me a very detailed answer to my question.

    By the way, this is an amazing forum, and all its participants are really making an effort to help others. Thank you to you all.
  2. jcsd
  3. Sep 25, 2008 #2


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    Staff: Mentor

    Welcome to PF.

    Newtons are a force, not an energy (and there is no "total force" over time) and so there is no direct relationship between the two. However, given the mechanical advantage required to get a person to generate 10,000 N of force, I'd say they'd probably be able to generate zero energy.

    Work (energy) is force times distance. Power is force times distance divided by time. That and mechanical advantage (divide or multiply by the ratio of the lever arms or gear ratios) are the only concepts you need here.
  4. Sep 26, 2008 #3
    Thank you very much for your answer.
    So let's take this for example:

    I pull a box which weighs 10 Kg, attached to a pulley.
    The pulley is at a height of 10 meters. And i pull the rope for 10 meters, so the box will be at a height of 10 meters. Then, lets assume that i do this process in a total of 1 hour. Then will that be right:

    (10 Kg * 10 meters)/3600 seconds = 100/3600

    Is that right so far? If yes, what will 100/3600 equal to? Newtons? How can i convert my answer to kWh? May you please give me an example of how you would have solved this? Thanks.
  5. Sep 26, 2008 #4


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    Staff: Mentor

    A kg is 9.8 N, so it is 10*9.8/3600=0.027 Joules/second (watts)

    Kwh and watt-hours are energy, watts is power. .027 watts for 1 hour is .027 watt-hours or .000027 kWh.
  6. Sep 26, 2008 #5


    Staff: Mentor

    An easier way to estimate this would be as follows:
    A 155lb person on a stationary bicycle burns about 352 kcal/hr. If you optimistically assume a 50% efficiency in the human converting food energy to mechanical energy and 50% efficiency converting the mechanical energy into electrical energy then you are generating 88 kcal/hr of electricity which is 102 watts.

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