How can "Earth Masses" and "Gravity" be different?

Crimson Kaim · Jul 17, 2016

Enlighten me please. AFAIK Gravity is the resulting force towards the mass of an object. The gravity of our earth is 9.81m/s. When browsing some of the planets and looking over their information (detailed surface scan complete) there are two informations:

1. Earth Mass
2. Gravity

I have noticed something I do not understand with a couple of planets. 1. and 2. are different. For example earth amsses is at 0.208 while hte gravity is 0.60G.

So if this trend continues we would achieve 1.0G while staying below 1.0 earth amsses (maybe 0.350?). Isn't gravity directly proportional to an object's mass?
Please tell me.

dynamicbob · Jul 17, 2016

Hydrogen worlds of earth size would be much lighter than our own planet, gravity comes from mass

Nagual · Jul 17, 2016

dynamicbob said:
Hydrogen worlds of earth size would be much lighter than our own planet, gravity comes from mass

What's got more mass a Hydrogen world with a mass of 20 or a Earth like world with a mass of 20?

Commander_Tin · Jul 17, 2016

It's a matter of density. A body of one earth mass that is half the size of the earth would have much higher gravity on the surface than earth does. Similarly, a body of one earth mass that is twice the size of earth would have lower surface gravity.

Gemai · Jul 17, 2016

Commander_Tin said:
It's a matter of density. A body of one earth mass that is half the size of the earth would have much higher gravity on the surface than earth does. Similarly, a body of one earth mass that is twice the size of earth would have much lower surface gravity.

+1

And in the core no gravity force would pull you as you are surrounded by equal mass pulling you in all directions. You won't live to see that outside of a small ball of fluids. Science!

Ziggy Stardust · Jul 17, 2016

The difference is the square of the distance from the mass.

Stahlkopp · Jul 17, 2016

Crimson Kaim said:
Enlighten me please. AFAIK Gravity is the resulting force towards the mass of an object. The gravity of our earth is 9.81m/s. When browsing some of the planets and looking over their information (detailed surface scan complete) there are two informations:

1. Earth Mass
2. Gravity

I have noticed something I do not understand with a couple of planets. 1. and 2. are different. For example earth amsses is at 0.208 while hte gravity is 0.60G.

So if this trend continues we would achieve 1.0G while staying below 1.0 earth amsses (maybe 0.350?). Isn't gravity directly proportional to an object's mass?
Please tell me.

Well, the force of gravity depends on the distance to the mass. So I guess the number they state is the 'gravity on surface' of the object.

In reality, we've discovered a few 'Super-Earths' by now. They're often much bigger than Earth and have more mass but the gravity on the surface is not proportional higher, in most cases it's only a little bit higher.

So there're two factor for the gravity on surface - the mass of the object ofc and the diameter of the object.

nerd-mode: off

GunnerBill · Jul 17, 2016

Commander_Tin said:
It's a matter of density. A body of one earth mass that is half the size of the earth would have much higher gravity on the surface than earth does. Similarly, a body of one earth mass that is twice the size of earth would have lower surface gravity.

This, yes exactly.

Gemai · Jul 17, 2016

Stahlkopp said:
So there're two factor for the gravity on surface - the mass of the object ofc and the diameter of the object.
nerd-mode: off

We could just use these stats to determine which planets have iron cores and how big they are. But who would do that...

Varrag · Jul 17, 2016

Ziggy Stardust said:
The difference is the square of the distance from the mass.

Yep.

OP: besides the mass, gravity on surface also depends on the radius of celestial body.

Noogillim · Jul 17, 2016

Inverse square law is at play, the Volume a planet takes up obviously alters the distance from the gravity centre to the surface of said planet.

The earths quoted gravity is an average, go to the top of Everest it's less as it is at the equator (spin induces bulge)

See what the game has done here? it has made you ask a question.

Ashnak · Jul 17, 2016

Gravity is (usually...) measured at surface level. Depending on the density of a body, two bodies of identical mass may have different diameters and therefore a different surface gravity.

If you want a formula:
F_g = G * m1 * m2 / r^2 for the force due to gravity. If you want the acceleration (or "gravity") on body 1 due to the mass of body 2, you'll get
a = F/m1 = G * m2 / r^2

Where G is the universal gravitational constant.

Using this for e.g. your spaceship (or SRV, or body) as m1 and the mass of a planet as m2, you see that the gravitationla accelration still depends on r^2, or the distance from the center of the planet (or rather, between the centers of the two bodies - but even a small planet is so much larger than your ship that you can safely do this approximation).

So, if you have a body of 1 earth mass, but only half the average density, it'll have twice the volume, or 1.26 times the radius. But its surface gravity will only be ~63% of earth's.

edit: Ninjas have been busy...

Flimley · Jul 17, 2016

Could be their relative density. So if two planets have the same raduis and one is much denser (increasing its mass also) than the other , it also affects it's gravity. ......... I think.
I'm sure someone will know for sure.

Flimley.

Ashnak · Jul 17, 2016

Stahlkopp said:
Well, the force of gravity depends on the distance to the mass. So I guess the number they state is the 'gravity on surface' of the object.

Next time you land on a planet, keep half an eye on the gravity indicator while going down. That 0.6 g's when entering orbital cruise might well go up quite a bit by the time you touch down on the pad.

IceFire122 · Jul 17, 2016

And here we are at the game teaching us something. Granted, it is not KSP with its orbital mechanics, but there are a couple of things to learn from ED.

drew · Jul 17, 2016

Density.

Cheers,

Drew.

Nagual · Jul 17, 2016

drew said:
Density.

Cheers,

Drew.

Oi! Who you callin' density?

Bomba Luigi · Jul 17, 2016

Science!

Thats all I know

Ziljan · Jul 17, 2016

Crimson Kaim said:
I have noticed something I do not understand with a couple of planets. 1. and 2. are different. For example earth amsses is at 0.208 while hte gravity is 0.60G.

So if this trend continues we would achieve 1.0G while staying below 1.0 earth amsses (maybe 0.350?). Isn't gravity directly proportional to an object's mass?
Please tell me.

Volume of a sphere increases as R³. However the force of gravity increases with 1/R².

Ergo, a planet with equal density will have 0.2 of the mass of Earth at ~3/5 radius of Earth.

At that distance the gravity would be scale as (0.2)x(5/3)² = ~0.6g

The density does play a role, but it's simple geometry at work here.

Koen · Jul 17, 2016

On top of this: the gravity of a planet being constant is a simplification.

Gravity on Earth is different depending on where you are. You weigh about 0.5% more at the poles than on the equator (part of that is due to spinning and not to gravity).

There are very small differences (on the order of 0.01% or less) in gravity due to differences in the local geology. For example, changes in the density of rock underneath you or the presence of mountains nearby can have a slight effect on the gravitational force.