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What is consumed when an object falls down ?

  1. Jul 31, 2013 #1
    When work is done using an energy source, say a battery, energy is consumed and the battery is able to do work till its chemical energy is consumed.
    Everytime an object falls down work is done by gravity ? Suppose everybody in the universe starts dropping objects into canyons, valleys and the oceans on earth, then why isn't it possible to exhaust gravitational energy.
     
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  3. Jul 31, 2013 #2

    adjacent

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    At the first place,Energy is required to move objects up the canyons valleys etc.
     
  4. Jul 31, 2013 #3
    If you drop something gravity does work on the object, but it also decreases its potential energy.
    The opposite would be lifting something up, which does increase its potential energy.
    That means that gravitational energy always reduces potential energy, so it does not get exhausted.
     
  5. Jul 31, 2013 #4
    Let's consider the case where people are pushing the earth around them down (no lifting objects to heights), say like standing on top of the grand canyon and all other canyons and blasting the earth down or people are standing around lakes, wells & oceans and pushing the earth down.
     
  6. Jul 31, 2013 #5

    adjacent

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    What do you mean by pushing the earth down?
     
  7. Jul 31, 2013 #6
    What was meant is nobody is lifting objects to heights and then pushing it down. People are just standing near large canyons, valleys etc, digging out large boulders and pushing it down.
     
  8. Jul 31, 2013 #7
    But the objects already have potential energy, if not they would not be up there (waterlevel = 0 potential energy).
    The objects would just fall and lose potential energy (and gain kinetical energy).
     
  9. Jul 31, 2013 #8

    A.T.

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    This will fill the canyons, and at some point you have equal potential around the globe.
     
  10. Jul 31, 2013 #9
    thus creating a "perfect" sphere?
     
  11. Jul 31, 2013 #10
    means value of g will change ?
     
  12. Jul 31, 2013 #11
    g will be constant everywheres, because everything will be reduced to the same height.
     
  13. Jul 31, 2013 #12

    phyzguy

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    I would argue that it is possible to exhaust gravitational potential energy. The universe started in a very uniform state with a very large amount of gravitational potential energy. As the matter "clumps up" into ever larger assemblages of mass, the usable gravitational potential energy is being used up. As the universe ages, more and more of the mass will be gathered into black holes, from which it is not possible to extract energy (at least once they spin down). This is more properly analyzed in terms of gravitational entropy. Penrose' book "The Road to Reality" discusses in detail how the universe was formed in a very low state of gravitational entropy and how the entropy is increasing enormously as matter clumps up to form black holes.
     
  14. Jul 31, 2013 #13
    because gravitational energy does not depend on a source, just like magnets you can keep coupling two magnets forever , you will have to spend some energy though to let them go away from each other , actually , when you drop something into earth , energy is somehow relieved , dropping something into earth or letting two magnets come into each other does not spend energy, but it actually gives energy
     
    Last edited: Jul 31, 2013
  15. Jul 31, 2013 #14

    Drakkith

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    Energy isn't really 'consumed'. Potential energy from the battery is simply transformed into other types of energy and moved elsewhere. Likewise, gravitational potential energy is transformed into kinetic energy as the object falls. After impact, ALL of the kinetic energy is transformed into heat and other types of energy. So what used to be gravitational potential energy is eventually transformed into other types and it takes work to give that object it's gravitational potential energy back.

    You could say that the objects gravitational energy, with respect to the earths surface, has been exhausted once it impacts the ground.
     
  16. Jul 31, 2013 #15
    I agree that gravitational potential energy is finally converted into heat. The question is shouldn't there be a corresponding decrease in the source of grav. pot. energy ? How is this manifested physically ?
     
  17. Jul 31, 2013 #16

    Drakkith

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    Let's say I drop a ball. The ball hits the ground and the kinetic energy is converted into heat and such. Until that energy escapes the system (the Earth and the ball) there is no decrease in the source. (If by source you mean gravity)

    But let's say some of that heat is radiated away from the Earth is has escaped our system. Now we have less energy, less mass, and less gravity than we had before.
     
  18. Aug 1, 2013 #17
    Let us consider a body of mass m at height h from ground at point A (say) initially at rest is falling under the action of gravity. If g be the acceleration due to gravity, then at point A,
    kinetic energy= 1/2(mv^2)=0 (because object is initially at rest)
    potential energy=mgh
    Therefore at point A total mechanical energy=mgh

    after covering distance x particle reaches at point B,which is h-x above the ground level.
    at this point velocity v* of object can be obtained by using equation,

    v*^2=u^2+ 2gx
    since u=0
    v*^2= 2gx
    and kinetic energy at point B is 1/2( m 2gx)=mgx
    potential energy = mg(h-x)=mgh- mgx
    therefore total energy=mgh

    now at ground velocity of object is given by
    v**^2=u^2+2gh (since object traversed the distance h)
    =2gh ( since u=0)
    kinetic energy at ground=1/2(m 2gh)
    =mgh
    potential energy=0 (since P E is measured from ground and h=0)
    therefore total energy= mgh

    Hence, from above we can see that total mechanical energy is constant throughout the motion of object.
    The force is to be done when object moves against the gravity, but your case is opposite of that. Means falling object is not against the gravity.
     
  19. Aug 1, 2013 #18

    adjacent

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    Is potential energy at sea level zero?
     
  20. Aug 1, 2013 #19

    russ_watters

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    The answer is simply no as can be obviously seen by the fact that you can lift the object back up and reset and repeat the scenario over and over again, forever.

    Consider rain.
     
  21. Aug 1, 2013 #20

    russ_watters

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    It is typically defined to be zero, but it doesn't have to be.
     
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