G unit and scale convertion.

[Ascensiam]

This challenge is more of a math challenge.

I need to have Kerbin\'s G (or Gee) converted to Earth G\'s

Then it can be compared to the planet diameter and then we can also figure out it\'s mass.

Through distance and speed of the mun we can also figure out the Mun\'s mass.

Such information is handy, or at least, a great curiosity.

[Kosmo-not]

Both G\'s for Kerbin and Earth are pretty much the same, actually.

http://kerbalspaceprogram.com/~kerbalsp/wiki/index.php?title=Kerbin

[Darkshadow]

G, is the universal gravitational constant, which is well constant. What you\'re asking for is the acceleration due to gravity, g which is variable with respect to distance.

[semininja]

Also, if you use WX_Echo\'s orbit calculator, found here, all of this information can be found in the data screens for celestial bodies.

[Cepheus]

G will be the same, everywhere. No matter what. It\'s a constant.

g, on the other hand, is acceleration due to gravity (roughly 9.8m/s/s on Earth.) While Kerbin is significantly smaller than Earth, it has similar gravitational acceleration.

[Iskierka]

Not similar; the same. Kerbin was scaled to maintain the same g, but allow for faster (time-wise) orbits.

Also to reduce mathematical precision errors.

[gabyalufix]

Why is trivially assumed true that the universal constant G would be the same on kerbin?

I mean--it is--but I\'m not sure it\'s the obvious question you guys make it out to be.

After all, it\'s a universal constant--it\'s only constant in a per-universe basis. Who says KSP occurs in our universe? You can, after all, simulate a universe with wildly divergent fundamental constants (not to mention fundamental laws). G could be set to anything you want.

[PeriapsisPrograde]

G is constant. If I recall correctly, it is the amount of force in N that 1 kg of mass exterts on a mass 1 meter away. G is very important to how gravity works. And, you know, orbits kinda count on gravity working... ;P

[gabyalufix]

G is constant in our universe. But if you\'re running simulation (or a game), you can set that sort of constant to be whatever you damn well please.

Orbits would still work, but the orbit sizes, speeds, various critical velocities, all that would be different.

In effect, changing that variable would be equivalent to shifting the relationship between Inertial mass and gravatic mass (edit: depending on which version you use to define 'mass', it might also 'rescale' your units of measure).

[Darkshadow]

G is constant in our universe. But if you\'re running simulation (or a game), you can set that sort of constant to be whatever you damn well please.

Orbits would still work, but the orbit sizes, speeds, various critical velocities, all that would be different.

In effect, changing that variable would be equivalent to shifting the relationship between Inertial mass and gravatic mass (edit: depending on which version you use to define 'mass', it might also 'rescale' your units of measure).

That is true, as all G does is scale Newton\'s law of gravity to equate to his 2nd law while using S.I. units as the kerbal Universe uses S.I units it makes sense that G is the same.

F=ma where F is in newtons, m in kilograms and a in meters per second per second

and F=GMm/r^2 where M in kilograms and r is meters

If you had a different universal constant, you could have the equations become: F=Kma and F=Mm/r^2 however as newton\'s second law is often taught first it naturally makes sense to keep it simpler and not have to introduce the concept of a scaling constant until Newton\'s law of gravity is taught.

[WX_Echo]

Also, if you use WX_Echo\'s orbit calculator, found here, all of this information can be found in the data screens for celestial bodies.

Thanks SN!

He\'s right; all of this data, and quite a bit more, is available in KSP Orbit Mechanic. Check out the 'Celestial Bodies' reference table for more information. FYI: you can even normalize your selected data to any celestial object in the game, and even a few in our solar system.

Also, as mentioned above, G is simply a constant of proportionality that allows us to match theoretical equations to empirical results. The value is constant everywhere in the universe, at least with our current knowledge of physics, and is usually not as meaningful as the gravitational parameter for a celestial body. This data is also provided in my calculator.

Hope this helps! Happy orbiting!

[Entroper]

Thanks SN!

He\'s right; all of this data, and quite a bit more, is available in KSP Orbit Mechanic. Check out the 'Celestial Bodies' reference table for more information. FYI: you can even normalize your selected data to any celestial object in the game, and even a few in our solar system.

Also, as mentioned above, G is simply a constant of proportionality that allows us to match theoretical equations to empirical results. The value is constant everywhere in the universe, at least with our current knowledge of physics, and is usually not as meaningful as the gravitational parameter for a celestial body. This data is also provided in my calculator.

Hope this helps! Happy orbiting!

To expand upon this, the gravitational parameter for a body is equal to G*M, where M is the mass of the body. If you know g, it is actually easier to calculate this value as a whole than it is to calculate M and then multiply by G. Observe:

F = ma (Newton\'s second law)

F = GMm/r^2 (Newton\'s law of universal gravitation)

Substitute for F:

ma = GMm/r^2

a = GM/r^2

Since acceleration at the surface is equal to g, and the distance between a mass at the surface and the body is R (the radius of the body) we substitute g for a and R for r:

g = GM/R^2

GM = gR^2

For Kerbin, g = 9.807 m/s^2 and R = 600 000 m.

[Ascensiam]

I know G is a universal constant, that is not what i meant.

What i meant is, what is the difference between the strength of the pull when standing still on Kerbin, compared to standing still on Earth, if the force on Earth is = 1, then what is it on Kerbin?

[Kryten]

The force on Kerbin is also exactly 1.

[Ascensiam]

The force on Kerbin is also exactly 1.

So Kerbin has the same gravity as Earth? Then it\'s mass must be tremendous.

[Kryten]

It\'s one of the base parameters that the Kerbol system was based around. And yes, this does mean that the mass of Kerbin is enormous for it\'s size-it\'s around 4 times denser than lead.

[WX_Echo]

I know G is a universal constant, that is not what i meant.

What i meant is, what is the difference between the strength of the pull when standing still on Kerbin, compared to standing still on Earth, if the force on Earth is = 1, then what is it on Kerbin?

You can lead a horse to water, but you can\'t make him drink.

From KSP Orbit Mechanic v1.2a:

[Entroper]

So Kerbin has the same gravity as Earth? Then it\'s mass must be tremendous.

Its radius is rather small, only 600 km compared to Earth\'s 6 378 km, so Earth is actually 113x more massive. Of course, the volume of Kerbin is even smaller in proportion to Earth\'s, so Kerbin is more dense.

[Gojira]

The Mün\'s gravity is also the same as the Moon\'s 8)

[closette]

Actually, orbital properties only tell us the product GM, so G could potentially be different and get around the high density planet problem. (Make G bigger in Kerbal universe -> M\'s get smaller).

Unless someone can come up with a Cavendish experiment on Kerbin, which you cannot do with the current physics engine.

We also don\'t really know what units the masses and thrusts of rocket parts are measured in when we read their VAB settings. Could be anywhere from kg to tonnes, Newtons to kN. And that means the sea-level atmospheric density could be anything from 0.01 to 10 kg/m³! Very clever of the developers to leave some room for interpretation...

[gabyalufix]

I believe all measurements are in SI units. Mass is kg, length is meters, thrust is newtons. I\'m fairly sure.

Kerbin rockets are extremely non-dense. That\'s why they all float. It\'s a little odd.

[LambdaCactus]

Kerbin rockets are extremely non-dense. That\'s why they all float. It\'s a little odd.

Either that or the silvery 'water' is very dense!

[Entroper]

I believe all measurements are in SI units. Mass is kg, length is meters, thrust is newtons. I\'m fairly sure.

Kerbin rockets are extremely non-dense. That\'s why they all float. It\'s a little odd.

The only one of these we know for certain is that length is actually in meters, because of the altimeter and velocity readouts. I used kg and N when I wrote the parts chart on the wiki, but that was just to avoid confusing people. AFAIK, there\'s nothing official that tells us the units of mass.

[WX_Echo]

The only one of these we know for certain is that length is actually in meters, because of the altimeter and velocity readouts. I used kg and N when I wrote the parts chart on the wiki, but that was just to avoid confusing people. AFAIK, there\'s nothing official that tells us the units of mass.

This is also my understanding. It would be nice to have proper units indicated for all parameters in the game.

[closette]

We do also know they are using seconds as well as meters, but the uncertainty in mass units remains, and as for solar system bodies in real life, we can only calculate the product GM from the orbital speeds of their satellites. (Specifically for a circular orbit of size R at a speed v, GM=Rv²).

Personally I don\'t mind being unsure of the mass units, for now. It allows for some room for interpretation, e.g. for the composition of their atmosphere, re-entry heating effects etc. It\'s incredibly difficult for the developers to come up with a self-consistent world while preserving playability. So I can live with some anomalies (such as Kerbin\'s density) and uncertainties (such as the speed of sound in their atmosphere) for now, at least until those things become important.