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Is the force of gravity constantly increassing?

  1. Oct 6, 2012 #1
    So I recently found out that gravity and mass are proportional and anything with mass has gravity, now I have another question, I am starting to wonder since the population of earth is constantly increasing it would also mean the mass on earth is also increasing (even a people who have died still have a mass and people being born daily is an increasing mass) so wouldnt this mean that gravity is increasing because the mass on earth is increasing?

    Let me try explain what I mean this way, plants take energy from things like the sun humans consume this energy in the form of calories, this causes an increase in mass and population.
     
  2. jcsd
  3. Oct 6, 2012 #2
    The mass from human bodies comes from carbon dioxide and water that's taken from the air and the ground by plants. (possibly with some animals in between)
    This doensn't add any mass to the earth.
     
  4. Oct 6, 2012 #3
    1: all that energy we get from the sun, is radiated back to space with infrared radiation... so we in the whole don't gain energy=weight.
    2: humans are a dramatically tiny precentage of the earths mass, so they have no real value there.
    3: as humans multiply, animals die... so when we infest the earth with our children.. animals will die, there is only a maximum amount of energy from the sun to sustain a maximum weight of biomass.
    4: all we eat comes from earth, so if you get 6 children that are 600kg, those 600 kg come from the ground, and that energy that came from the sun radiates again to space... so the whole mass stays the same
    5: the earth is actually LOOSING mass, because of massive amounts of hydrogen and other gases being emitted from the sea also carbon etc, they are large deposites of stored gas and may cause the end of humanity, as studies show this has happened before. The mass we gain from meteorites is less than what we loose from gas.
    6: you mean the mass of earth, not the force of gravity, the force of gravity relative to a things mass has stayd the same since measurments have started.

    and that's about it.
     
  5. Oct 6, 2012 #4

    mfb

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    To add some numbers to Jarfis points (* are references):

    59736 00000 00000 00000 00000 kg - the approximate mass of earth
    -----------...56000 00000 00000 kg - the estimated* total mass of all living species apart from bacteria (bacteria probably weight more than that*)
    ---------------..50 00000 00000 kg - the total mass of all humans (assuming ~70kg/human)
    Anyway, living organisms just eat things on earth and die on earth, they do not change the total mass at all. Let's see which processes can actually change the total energy or mass content:

    ~3kg/s loss of hydrogen*
    ~45TW loss of geothermal energy (see references here), which can be viewed as 0.0005kg/s
    0.1 kg/s upper limit for the mass loss due to spaceflight (assuming 100 rocket launches per year with ~30 tons to orbit each
    Wild estimates for meteorites, but those numbers were all below 1kg/s.
     
    Last edited: Oct 6, 2012
  6. Oct 6, 2012 #5
    What about mass gained from solar wind that strikes the earth and gets stuck here?
     
  7. Oct 6, 2012 #6
    Yes but I doupt that the mass of a wind of protons and electrons is going to be much... if we want to go all the way we do the following:

    1: find the sum energy that enters the definition of eart(line between earth and space) this can be energy in the form of light, meteorites, intergalatic radiation or as you said solar winds.
    2: find the sum energy of all that leaves earth, this can be sattelites, space trash, rocket fuel, hydrogen, carbon and all those different gasses, light, and infrared radiation.
    3: find delta energy 1&2 in one year and you have the energy that earth uses or gains each year.
    4: convert this energy to mass using einsteins equation, than you see how dramatic earths weight loss or gain is.

    I am guessing that earth is going to be loosing mass in the whole since gas leaving it is in big proportions.
     
    Last edited: Oct 6, 2012
  8. Oct 6, 2012 #7

    K^2

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    Earth is getting heavier over time. The biggest contribution to Earth's mass change is space dust Earth picks up. However, the increase is so small compared to the total mass of Earth that it's not significant in terms of gravity.
     
  9. Oct 6, 2012 #8

    mfb

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    2kg/s (first number from Wikipedia), but most particles are deflected by the magnetic field.

    @K^2: See the posts abov please.

    Oh, and 3kg/s over 5 billion years is 0.00000008 earth masses :tongue:
     
  10. Oct 6, 2012 #9
    As I said previously, there's also mass losses, and I remember reading the sum of earths mass is decresing.
     
  11. Oct 7, 2012 #10

    K^2

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    Yes, I underestimated the amount of loss. The mass gained due to space debris and dust adds up to 1.2kg/s, which isn't enough to offset the 3kg/s dominant factor in the loss.

    Any way to estimate how much is getting deflected?
     
  12. Oct 7, 2012 #11
    Earth's gravity is actually really non-constant, but not because of increasing mass.

    The earth isn't a perfect sphere so your latitude has a pretty big effect on the total gravity you experience (the greatest is at the poles). To the point without exact measurement at your location you actually can't be sure what gravity is past 9.8 m/s^2.

    Latitude has an effect of .03 m/s^2. 9.81 is the value at mean sea level at the equator. So guessing between 9.81-9.84 are a good estimate of the range on earth.

    So in general, there are even more effects out at the .000X because of non-uniform density and elevation.

    There actually is a formula based on latitude to determine gravity within .001 of of your location.
     
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