Saturday is Sexy… Sunday is Movies Night
This was a great question (reworded for clarity):
“I understand that the gravitational force that the Earth pulls on the moon depends on the mass of the Earth. So, as the human population increases does this mean that we will eventually pull the moon closer to the Earth?”
Let’s look at this in different parts. First, what is the mass of all the people on the Earth? As I am writing this, there are about 7 billion humans living on Earth.
If I know the average mass of a person, I can get the mass of all the people. Here’s where we guess. Well, not a completely random guess. No, it will be a well aimed guess. An average human male is maybe 160 pounds (70 kg) and a female is about 110 pounds ( 50 kg). I suspect those are a little high for all of the adults. I would guess that adults in the USA are a bit more massive than other parts of the world. Now, what about children? I could say that there about as many men as women. That would put the average mass somewhere around 60 kg. If I take into account children, maybe this average mass is around 40 kg.
Now that I have an estimate for the average mass of a human, the total human population mass is just going to be the total number of humans multiplied by the average human mass. This would give a mass of 280 billion kg (2.8 x 109 kg). That’s a large mass, but how does it compare to the mass of the Earth? At about 6 x 1024kg, all the humans are just a tiny fraction of the total mass (4.7 x 10-14 %).
Ok, there is another part to this question. Does the mass of the stuff on Earth change? By “stuff” I mean all the living things and consumable things like air and water. In short, the answer is no. Where do humans come from? Or perhaps I should ask where the mass that makes up humans comes from? As a person grows or as new people are created, the mass for this new material comes from three sources: air, water, and food. Actually, I’m not too sure how much of our material comes from the air but it could be a small contributing factor.
But where does the food come from? If everyone was a vegetarian, the food would come from plants. But where does the mass of plants come from? Most of the plant mass comes from the air? Yes, the air. Plants take in carbon dioxide and produce oxygen. They save the carbon (and water and other stuff) to use as building blocks for growing. It might seem strange, but it’s true.
That means that indirectly at least, the mass of the human population comes from the air. When people die, they decompose and produce more carbon dioxide. It’s an endless cycle. Just about all of the mass comes from something that is already on the Earth. Yes, I said “just about”.
Does any mass leave the Earth? Does any mass added to the Earth? The answer for both questions is yes.
When does the Earth lose mass? First, there is loss of gas in the atmosphere. Think of our air as a bunch of gas particles bouncing around. That’s essentially what is happening. Some of these particles of gas (oxygen or nitrogen molecules) are going quite a bit faster than other particles. If the particle is going fast enough and it is near the top of the atmosphere, it can escape Earth’s gravitational influence. This does happen, but the effect is quite small. The other way for the Earth to lose mass is when humans send objects into space. Again, the total mass of all the man made space objects is quite small.
The Earth also gains mass. According to NASA, there are about 100 tons of meteoroids hitting the Earth each day. Over a year, this would be 3 x 107 kg. It would take about 100 years worth of meteors to equal the mass of the human population. However, doubling something super small is something still super small.
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