# Gravity -- Does gravity depends on atmosphere?

1. Jan 22, 2015

### san D

Does gravity depends on atmosphere

2. Jan 22, 2015

### jambaugh

No.

3. Jan 22, 2015

### san D

Then on what factors it depends...??

4. Jan 22, 2015

### DaveC426913

Mass and mass only.

(Now gravitational force, on the other hand, depends on mass and distance)

5. Jan 22, 2015

### Bandersnatch

The answer could be just as well 'yes'. It all depends on what you actually mean.

Why won't you try being a bit more descriptive? What exactly is it you want to find?

6. Jan 22, 2015

### phinds

My answer would be no, it's the other way around. The atmosphere depends on gravity. sanD, you don't seem to be putting much (ANY actually) research into finding this out on your own. This is not one of those forums where you ask trivial questions and someone tells you the answer, it's a forum where we try to help people figure out how to get answers on their own. The first thing to do on your own for a question like this is type the question into Google and see what pops up. If that doesn't quite give you want you want, come here with a more focused question.

7. Jan 22, 2015

### Nihilist

A body on a planetary surface will experience a different gravitational acceleration depending on what radius they are from the gravitational source's mass centre (core). This body will also likely be under atmospheric pressure, with a summed force in the same direction as the gravitational acceleration. Does this atmospheric force compound the gravitational force or will the atmospheric force have a net of zero?

8. Jan 23, 2015

### mp3car

I'm going to take a guess at what the OP may be thinking about... which may be related to the different meanings of mass vs. weight...

When you "weigh" something in the conventional sense, in open air, you're actually not "weighing it" as much as you are determining how much "heavier" it is than air (or comparing the differences between the force on the atmosphere due to gravity and the force on the object due to gravity). It's a little easier to see what I'm getting at if you take the example of if we take a scale of infinite mass to the bottom of the ocean. If you placed a rock on a scale, you don't get the same "weight" as you do on land, even though gravity is (essentially) the same.

Like i said, I am just taking a guess of what the op may have had in mind when they asked the question...

9. Jan 23, 2015

### davenn

no, but the opposite is true when it comes to a planet having an atmosphere.
Without significant mass, the planetary body wont have a strong enough gravity to retain an atmosphere

10. Jan 23, 2015

### phinds

Read post #6. You're a day late

11. Jan 23, 2015

### davenn

well at least I gave you good backup ;)

Dave

12. Jan 23, 2015

### JLowe

Perhaps he wants to know if the effect of gravity feels stronger with greater atmospheric pressure. I'm not sure it works like that, I think a strong gravitational pull would crush you into the ground whereas a high atmospheric pressure would be more like an implosion. Or am I wrong?

13. Jan 23, 2015

### phinds

You are right. Well, a crushing rather than an implosion but I see that you were thinking in the right direction.

In any case, it gets boring trying to figure out what some random posted comes and asks and then goes away and you never hear from them again. Maybe he'll come back to say what he really wants to know but don't hold your breath.

14. Jan 24, 2015

### san D

Confused about this the gravity changes when we move out of the surface of earth at the same time we can observe atmosphere also changes that's why i confused

15. Jan 24, 2015

### phinds

Did you think the Earth's atmosphere just goes on forever into the universe? Of course it tapers off as you get higher, as does the force of gravity. What is confusing about that?

16. Jan 24, 2015

### san D

Thank you

Last edited by a moderator: Jan 29, 2015
17. Jan 29, 2015

### san D

I will agree that gravity depends on mass only ...then the gravity of uranus must be greater than earth's gravity because uranus is bigger than earth but it's not why...?

Last edited by a moderator: Jan 29, 2015
18. Jan 29, 2015

### JLowe

That is probably surface gravity you're thinking about, which can be greater for a less massive object because the surface is farther away from the center of gravity in the massive object.

19. Jan 29, 2015

### Bandersnatch

Gravity doesn't depend on mass only, but also on distance from the source. The gravitational force equation shows that clearly:
$$F=G\frac{Mm}{r^2}$$
(G is the only constant here)
Dividing both sides by m(test particle mass) you get the gravitational acceleration:
$$a=G\frac{M}{r^2}$$
As long as the two objects you're comparing have the same density, the larger an object is(greater r), the greater its gravity because as you increase radius the mass increases faster than the square of radius:
$$M=V\rho$$
where ρ is the density and V is the volume of a sphere
$$V=4/3 \pi r^3$$
Combining the above you get $$a=4/3G\pi r \rho$$

If you'll make one of the objects less dense, its surface gravity will fall down. As long as you make the density fall by the same fraction as you increase the radius, the surface gravity will stay the same.

With Uranus, its density is 4.3 times lower than Earth's while its radius is 4 times larger.

20. Jan 29, 2015

### DaveC426913

As I said: gravity depends only on mass. The gravitational force experienced at some point depends additionally on its distance from the mass. A fine distinction perhaps, but a distinction nonetheless.