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Source: Earth weighs about 13,170,000,000,000,000,000,000,000 pounds (or 5,974,000,000,000,000,000,000,000 kilograms). Since Earth is too big to be placed on a scale, scientists use mathematics and the laws of gravity to figure out Earth's weight.

So, this seems to be a constant. But I guess it doesn't consider the things that are on the earth, the trees, buildings, people, animals, water etc...

So, are all the above mentioned things' mass is too small or negligible to be left off?

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    $\begingroup$ The mass of the Earth is calculated by measuring how fast objects orbit it, so that mass includes, trees, plants, even bugs. That's the easiest way to measure the Earth's mass. An estimate of the mass of all trees, for example, is quite a bit more difficult and I'd wager, a good deal less accurate. $\endgroup$
    – userLTK
    Commented Sep 19, 2015 at 10:08
  • $\begingroup$ If you like this question you may also enjoy reading this Phys.SE post. $\endgroup$
    – Qmechanic ♦
    Commented Sep 19, 2015 at 15:37
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    $\begingroup$ Add? We didn't all beam down from the Enterprise. $\endgroup$
    – Solomon Slow
    Commented Sep 19, 2015 at 17:08
  • $\begingroup$ the things on earth "the trees, buildings, people, animals, water etc..." are made from the particles present on Earth itself, if u don't count those little asteroids hitting sometimes...... $\endgroup$
    – Shubham
    Commented Sep 19, 2015 at 17:35
  • $\begingroup$ @Qmechanic that was a good read. And come on, if such questions were given as homeworks, Indian Education system would reach a new high, alas! it's not the case. This question just cropped up in my mind out of curiosity. My age might have lead you to that assumption. I thought the weight was only of Earth's surface and it's core, anyway the answers cleared up my trivial doubts. Thanks. $\endgroup$
    – Amit Joki
    Commented Sep 20, 2015 at 2:38

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As all living cells take their matter from the earth, the overall mass stays constant. The earth is (mostly) a closed system with only very few exceptions:

  • Gases, that are light enough to escape the earth atmosphere (I think this can happen to helium)
  • The stuff humans send into space using rockets
  • A (tiny) bit of mass is added to earth by the sun's energy (not much but it follows from Einsteins relativity). However if the system is in equilibrium, the energy leaving the earth is equal to the energy reaching earth. More or less this will be the case.

By humans being born or dying the mass however does not change.

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  • $\begingroup$ Regarding added mass from solar energy: the Earth also radiates/reflects away as much energy as it receives from the sun (otherwise it would continually heat up) and so the total mass is actually unchanged in the end, in that regard. $\endgroup$
    – Asher
    Commented Sep 19, 2015 at 16:01
  • $\begingroup$ Well that's right if the system is in equilibrium. Will add that to the answer. $\endgroup$
    – Hagadol
    Commented Sep 19, 2015 at 17:15
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That depends on how you found that weight.

If it is calculated from the gravitational pull, then all things on earth are included since they add to the mass.

If it is calculated through geological measurements then maybe they are not included.

In any case, try to multiply 80 kg or so for an average human with 6 billion. Try to add an estimate for anything else you wish to add. In any case your number is way smaller than any significant value. I am pretty sure that counts as negligible.

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Because of conservation of mass, the fact that things on Earth are composed of trees, buildings, etc, doesn't matter - the only time the mass of the Earth will change is if things physically leave the Earth.

According to this article the major change in the mass of Earth is due to about 10$^5$ kg of Hydrogen and Helium escaping from the atmosphere every year, whilst 4$\cdot$10$^4$ kg of space dust falls onto the Earth. So overall the change in mass of the Earth is a decrease of around 6$\cdot$10$^4$ kg per year.

Now there's actually a fairly large uncertainty in the mass of the Earth due to the difficulty of measuring G. But this nature article suggests the uncertainty in the mass of the Earth is around 10$^{20}$ kg. So overall the change in the mass of the Earth is completely negligible compared to the accuracy with which we can measure the Earth's mass.

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The atoms that makes up life on Earth comes from the earth itself. It included in that estimate.

Do you want to make an estimate of the mass of all biomass?

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