# More human population = greater mass?

1. May 25, 2012

### blofse

Hi everyone!

This is my first post on this forum so please, if I have missed something please let me know!

I have a lot of questions to ask but this is the main/first question in my mind right now!

As I understand things, everything in the universe has mass (or anti-mass) and everything that has mass exerts gravity (depending on density). Every human has mass and a very low level of density meaning they exert a small amount of gravity and they have a small amount of weight.

So with all that in mind, as the population of humans increases exponentially, does the earth either weigh more, less and does it shrink or contract due to humans?

The way I see it, although we can't have any more material mass on earth, what about biological?

Please start my education and stop my ignorance!

2. May 25, 2012

### Whovian

First of all, note that biological mass is material mass.

But setting that aside, the mass to form a baby and the mass to grow must come from somewhere, in most cases, food. This food would have already been on Earth, and so its mass as a whole doesn't change.

3. May 25, 2012

### blofse

Ok thanks for the correction and speedy reply.

Good point about food. In fact, its a rather annoyingly simple answer. I shall go and have a think about this - one thing, don't we convert some of the food to energy (which has no mass)?

4. May 25, 2012

### Whovian

Or does it? I'm pretty sure that GR predicts that energy basically does have mass, as modeled by $E=m\cdot c^2$

5. May 25, 2012

### DaleSwanson

When food is converted into energy its chemical bonds are broken (or new ones are formed) and energy is released. Those bonds have some very slight mass, and the resulting energy has the same very slight mass.

Going back to your original post, I'd like to point out that it's total mass and distance that determines force of gravity. Google tells me that the Earth's average density is about 5.5 g/cm3, compared to a human's density of about 1 g/cm3 (ie about the same as water). You'll note that the Earth has much more than 5.5 times the force of gravity than a human body. This is because of its much greater total mass, not density.

At the risk of confusion, if the Earth were compressed so its mass remained constant but its density went up, the force of gravity would increase. The reason is that the distance would go down. As mentioned above distance and mass are the two factors that determine force of gravity. Making the Earth smaller, but with the same mass (ie more dense) would make you closer to each atom of the Earth, and thus increase the force of gravity.

6. May 29, 2012

### blofse

Ok.
So just to be clear, and to check my understanding - when food is grown from the soil, that is made of all the mass from the materials from the earth. We then eat that food, convert most to energy and the rest to waste mass.

At no point along this chain is any mass lost, i.e. it is constant thoughout.

Is this right?

What you guys are saying is life on earth has NO impact on the earths size. In fact, neither does anything else - except external factors such as astroids/solar energy etc.

If that's true then good. Got it :-)

Thanks. That's a good reminder of school physics makes me realise I have forgotten a few facts - whoops!

Another question your going to like - does that mean then that if there is a town/country with a very high population density, does this have a greater gravity then other like-for-like land masses? Obviously a like-for-like may not actually exist, but in theory if it was the same shape, made of the same matieral etc etc....
Or is there something else I have missed...

7. May 29, 2012

### Staff: Mentor

Yes. One note: one of the materials used is carbon dioxide, which technically is not from soil, but from the air. But it doesn't change the overall mas of the Earth, as when we talk about mass of the Earth, we include mass of the air.

8. May 29, 2012

### blofse

Ok so that means we have a finite population we can reach, as there is a finite (well, ~constant) amount of mass on earth.

Presuming we can't leave the earth that is....

9. May 29, 2012

### cepheid

Staff Emeritus
The original post should have just said, "everything that has mass exerts gravity" without the parenthetical remark about density. Density doesn't have anything to do with it. The amount of gravitational force that a body feels depends upon that body's mass, the mass of the thing it is gravitationally interacting with, and the distance between it and that thing.

10. May 29, 2012

### blofse

It was more a question of gravity that a human body creates rather than what it feels. So in that context, density does matter - does'nt it?

I am not sure I am missing something here - so please bear with me!

11. May 29, 2012

### cepheid

Staff Emeritus
No, density matters neither for gravity exerted nor felt. Look up Newton's Universal Law of Gravitation.

12. May 29, 2012

### blofse

Ok thanks - will do.

13. Jun 2, 2012

### jduster

The law of conservation of mass is clear that humans may increase in mass, individually and collectively, but the resources do come from the Earth. So no mass is added, it is just moved around.