Gravity and gases

[mab]

Gravity pulls every thing (matter) towards itself (solid,liquids and gases).

My question is that gravity is like magnet for matter,If the body has larger mass it ‘s gravitation pull is greater e.g. earth gravity is greater than moon’s ,therefore moon revolves around earth.

But in the case of solids, liquids and gases the situation is puzzling for me, because gases are lightest still solid and liquids are more in the control of earth’s gravity .Can anyone explain the reason?

[VendingMenace]

Well, first off, the earth and the moon revolve around eachother. It is just that the center of mass for this system lies quite close to the earth -- so it is conveinent for us to desribe it as the moon revolving around the earth. (though i guess it is all realitive and you could say the earth revolves around the moon -- but that is neither here nor there.) The point being that the moon and the earth both exert gravitational forces on eachother.

 

But onto your real question

 

I am guessing that you are asking something like "why does the air lie above the water and the land?" Wich might be a tricky question at first glance. However, it is akin to the question of "why do air bubbles rise in water?" For wich there is a good answer. This answer is; water is more dense than air. What that means, of course, is that in a given volume, there is more mass of water (or other liquids) than air (or other gasses). Since gravitational forces are a function of mass, we can see that for this given amount of volume the earth pulls harder on the liquid than on the gas.

 

This is all well and good, but just because the earth pulls harder on liquids -- does it nessisariy follow that liquids must lie closer to the earth? Actually, yes. This has to do with the concept of energy. Gravitational potential energy is realted to two things. How hard the earth pulls on something (which we find by taking the acceleration of earth gravity and multipliying it by the mass of the object wich we are considering. this lets the energy scale with the mass of the object) and how high that object is. In the end this quantity of potential energy looks like this;

 

Energy = (acceleration of earths gravitational feild)*(mass of object)*(height of the object)

 

Ok, so the last thing we need to know, to answer your question is this; systems will always align themselves such that they have the lowest possible energy.

 

OK, now we know all we need to answer your question, so lets take a look at it.

 

Let us say that we have a given volume of gas and the same amount of liquid. ANd they are in a box wich is sitting on the ground. Both of these "objects" (liquid and gas) have a mass associated with them. The mass of the liquid is greater than that of the gas, by nessesity (since the liquid is more dense than the gas, and we have an equal volume of the two). The sytem inside the box now must oder itself such that it has the least amount of energy possible. We can see that by looking at the equation above for potential energy, that the liquid will have the most energy associated with it -- should it be at the same heighth as the gas. Thus, the liquid must reside at lower elevations, if the energy of the system is to be the lowest possible.

 

 

 

Well, i doubt that explination is 100% correct or rigorous, but i think that it should at least get you thinking in he right direction. Hopefully if i am wrong, someone more in the know than I will correct me though. But at least for now, that is what i am thinking. Hope it helps. ;D

[mab]

I was thinking that if spining of planets may be one of the reasons,behind for gases being in the outer most layers of atmosphere.

similar to the spinner in washing machine .

I read somewhere that,"The spinning force acts against gravity and causes many planets to bulge out more around their equators".

Can you explain this?

[swansont]

I was thinking that if spining of planets may be one of the reasons' date='behind for gases being in the outer most layers of atmosphere.

similar to the spinner in washing machine .

I read somewhere that,"The spinning force acts against gravity and causes many planets to bulge out more around their equators".

Can you explain this?[/quote']

 

In order for an object to move in a circle, there must be some force acting toward the center (centripetal force). This can be provided by gravity.

 

Consider a non-rotating planet - you will have basically a sphere, with all of the forces of contact between the material, and gravity, balancing out. If the planet starts rotating, though, there's nothing left to make it move in a circle, unless you reduce the contact force (compression). The surface wants to travel in a straight line (not straight out) and in doing so, the planet would expand. It stops expanding when the net force is equal to the centripetal force necessary for circular motion.