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Is earth's weight constant?

  1. May 22, 2009 #1
    i know this question is in poor taste but still:
    is earth's weight constant?
  2. jcsd
  3. May 22, 2009 #2


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    Re: weight

    Weight? Its mass is more or less constant, might gain a small amount when something hits it etc. The acceleration due to gravity is slightly different at different points. For instance if you measured g on the side of the large mountain you would notice that the mass of the mountain moves the centre of mass of the earth slightly.
  4. May 22, 2009 #3


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    Re: weight

    You'll need to clarify your question. If you mean is the Earth's mass constant, then yes, on human time-scales it is constant. On a geological time-scale of hundreds of millions of years and such, it is basically constant, there's impacts from things such as meteors that increase the mass ever so slightly but still, it's so little that it's basically uninteresting.

    If you mean a mass' acceleration due to the earth depending on where you are, no it isn't constant. If you travel to the moon, your acceleration due to the earth drops off dramatically obviously. If you mean your acceleration at different points on the surface, there is a slight difference. If you measure your weight where you live and then travel down to, say, Antarctica, your weight will be slightly different most likely. This is because Earth is not a constant distribution of mass. Actually, that is one method of discovering oil; the acceleration of a body at a point on earth that is relatively solid underneath it differs from a point on earth where there is a large mass of oil hiding in the Earth.
  5. May 22, 2009 #4
    Re: weight

    If the question is about the force of gravity which the earth is subject to, due to other massive objects such as the sun, then would it be fair to say no, the earth's weight varies slightly? The force of gravity depends on distance, and the earth's distance from other bodies isn't perfectly constant. In particular, it's orbit isn't a perfect circle, so the force required to hold it at rest should be greater when the earth is closer to the sun.
  6. May 22, 2009 #5
    Re: weight

    Would a better way to phrase this be: "the force required to counteract acceleration due to gravity"?
  7. May 22, 2009 #6
    Re: weight

    That difference is really small. I recall Carl Sagan writing in Cosmos that the obstetrician who delivers you exerts a greater gravitational pull on you than does Mars.
  8. May 23, 2009 #7
    Re: weight

    From http://curious.astro.cornell.edu/question.php?number=470 [Broken]
    Estimates for the mass of material that falls on Earth each year range from 37,000-78,000 tons. Most of this mass would come from dust-sized particles.​

    Such a wide variance in estimates is a good indication that no one really knows the true figure.
    Last edited by a moderator: May 4, 2017
  9. May 23, 2009 #8
    Re: weight

    Taking the radius of the earth's orbit from Wikipedia, and values for G and the masses of earth and sun from Google Calculator, I get approximately

    aphelion: 3.427 * 10^22 N
    perihelion: 3.664 * 10^22 N

    So a difference of about 2.37 * 10^21 N. Does that sound reasonable?
  10. May 23, 2009 #9
    Re: weight

    I think the earth can lose mass as well as gain it.Light atoms such as hydrogen and helium can reach escape velocity and helium is constantly being injected into the atmosphere as a result of alpha decay, particularly from radon gas.
  11. May 23, 2009 #10
    Re: weight

    My thoughts: Hydrogen, helium, or any atom for that matter, that manages to worm itself into the earth's upper atmosphere by it's own does not have the kinetic energy to permanently leave the gravitational influence of our planet.

    That is, I seriously doubt that if I were to release hydrogen into the air from my home that any of it would end-up in interstellar space.

    But I could be wrong here.
  12. May 24, 2009 #11
    Re: weight

    My thinking is that some of those molecules which lie at the higher energy end of the Maxwell Boltzman distribution curve can reach or exceed escape velocity and and if at any instant they are in the right location and moving with the right trajectory they can indeed escape.Escape will be more likely with the smaller mass atoms since at any given temperature their average velocities will be higher.
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