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Energy required to the Moons orbit

  1. Dec 6, 2009 #1
    How much energy would it require (per second or per orbit) to keep the moon in orbit, if gravity did not exist?

    Pretend the gravity from Earth did not exist, and the moons still should orbit like it does.

    Does it exist a equation to calculate that?
     
  2. jcsd
  3. Dec 6, 2009 #2
    I don't think such an equation will ever exist seems as our universe (that is the visible universe) we live in is dependant on gravity to keep everything in the cosmos in order. But I guess there is a chance that my assumption is wrong and their maybe some genius who may determine a formula and thus answer your question :)
     
  4. Dec 6, 2009 #3

    russ_watters

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    w=f*d
    d=0, therefore w=0

    Keeping an object moving in a circular path, whether via an orbit or just tying a string to it and pulling on it, does not require energy.
     
  5. Dec 6, 2009 #4
    In that case you will have energy "on that string" pulling outwards
    How much energy on that string per second ?'
    Would the object lose speed ?
     
  6. Dec 6, 2009 #5

    russ_watters

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    No, you won't have energy lost on the string. The force is perpendicular to the direction of motion.
     
  7. Dec 6, 2009 #6

    Matterwave

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    As I mentioned in your other post, P=F dot v

    F and v are perpendicular for circular orbits, and thus there is no power! No power means no change in energy since P=dE/dt=0

    The only thing that matters with energy is Change in energy. You can assign any arbitrary energy for the system, but if it doesn't change, then it doesn't matter.
     
  8. Dec 6, 2009 #7

    marcus

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    Would you like to know the force required to keep the moon in orbit?

    What units of force do you like to use?

    In metric, the unit of force is usually the Newton.

    In other systems force can be expressed in pounds-of-force, or tons-of-force, or some other way. If you would like to know the force required to keep the moon in orbit, would you like to know it in terms of tons-force?

    There certainly is a formula! You just have to say what units of force you prefer.
     
  9. Dec 6, 2009 #8

    marcus

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    If gravity was going to be turned off, and you tied the two bodies together with a long string, then the string would have to be 240 thousand miles long. (Simplifying the orbit to be circular)

    And the force I am talking about would be the tension on the string that holds the two together.

    It doesn't take any energy, because energy is work, and nobody has to do any work to keep the two bodies whirling around, attached together by a piece of ideal string. But there is this force, this tension in the string.

    Try putting this into the google search window:

    G*(mass of earth)*(mass of moon)/(240000 miles)^2

    type it in, just like that, and press "return"

    it will tell you 2 x 1020 newtons.

    That is how much force. If you want it in pounds you can, instead, type in

    G*(mass of earth)*(mass of moon)/(240000 miles)^2 in pounds

    and then divide the answer by 2000 to get it in tons, because there are 2000 pounds in a ton.
     
    Last edited: Dec 6, 2009
  10. Dec 6, 2009 #9

    russ_watters

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    For the force, you can just use Newton's law of gravitation.


    [edit: you beat me by a few seconds...]
     
  11. Dec 6, 2009 #10

    marcus

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    Great minds, Russ :biggrin:
     
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