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I was wondering :

does the weight on the planet earth is equal over the years ?

meaning :

all the people , ground , water ,gas.

does the weight stays the same over the years ?

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2 Answers

up vote 3 down vote accepted

This answer is very similar to Adam's, though I come to the opposite conclusion i.e. that earth loses mass over time.

According to the Scientific American article the earth loses about 3kg of hydrogen per second, and I make that about $10^8$ kg per year.

According to this article the Earth gains about $3 \times 10^7$ kg per year from meteors (mostly extremely small ones).

So unless there are other sources of weight loss or gain that I haven't thought of, I reckon the Earth gets lighter by about $7 \times 10^7$ kg per year.

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loses hydrogen to where ? –  Royi Namir Jun 22 '12 at 19:00
    
you say getting lighter...adam says heavier.... –  Royi Namir Jun 22 '12 at 19:01
    
John, interesting article, I hadn't seen that one before. I understand the mechanisms they discuss, but it surprises me that they think there is enough momentum behind the molecules and atoms to eject them out of Earths gravitational well. I suppose they just need enough to get caught in the solar winds... huh, interesting. –  AdamRedwine Jun 22 '12 at 19:08
    
Royi, I once had a homework problem to determine the solar energy incident on the Earth. Every person in the class got a different answer and the teacher's answer was different still. Welcome to physics. –  AdamRedwine Jun 22 '12 at 19:09
1  
@RoyiNamir: the weight loss/gain is basically insignificant compared to the weight of the Earth, and any figures Adam and I come up with are going to be very approximate. I think Adam would probably agree with me the weight change is zero within experimental error. Re the hydrogen loss, hydrogen is a very light gas and it can diffuse to the very top of the atmosphere where it gets removed by (as Adam says) the solar wind. –  John Rennie Jun 22 '12 at 19:32
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Like many questions involving large complex systems, there is a fractal nature to this question. The question is similar to how long is the coast line of a country, do you count the land exposed by tide? What about the waves?

In this case, there are things to consider such as, do you count the mass of satelites? What about the equipment left on the Moon? How about the atmosphere? How you answer these will depend on what you want out of the calculation.

Some of these things seem pretty straightforward. The mass of meteorites entering the atmosphere is actually quite substantial; these represent an obvious increase in mass. Satellites, on the other hand, represent a pretty obvious loss of mass. Given that there are "only" about 10,000 satellites in orbit, they represent a loss of less than about a single year's worth of meteorites.

Slightly less obvious, I would include the mass of the atmosphere and also of the airplanes and other objects contained within it. The atmosphere may seem etherial, but it is quite massive. It does not, however, change with respect to contributing to the mass of the planet.

On the whole, it appears that the mass of the Earth increases over time with the increase primarily due to meteorites.

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so if all sattelite and birds etc are landing.... the weight is the same except for meteorites....right ? –  Royi Namir Jun 22 '12 at 18:53
    
Again, depends on what level of detail you like. Do you want a number that is accurate to the giga-ton? to the ton? to the kilo? to the gram? If you go to that level of detail, the mass is changing constantly for a huge variety of reasons. the less resolution you take in your answer, the more stable it is. –  AdamRedwine Jun 22 '12 at 19:03
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