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fliguana t1_ixurxma wrote

"gravity" (field) of the planed is determined by its mass, nothing else.

The weight of an object is determined by all gravity fields and acceleration of its motion.

Solar gravity can be detected on earth, because tides are higher during new/full moon events. But acceleration plays even greater role, objects in equator weigh less than those on poles.

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haysoos2 t1_ixwhb16 wrote

Not actually just the mass. The distance between the center of mass to your center of mass has an effect as well. Therefore a dense planet will have a different surface gravity than a planet of the same mass that is built of lower density material.

If you know the diameter of the planet, and its average density it's fairly simple to calculate its surface gravity.

Take the planet's diameter (in miles), multiply by the average density (in g/cm3 or kg/l), and multiply that by 0.0000229

This will give you the surface gravity in G (where Earth's gravity = 1)

Earth has a diameter of about 8000 miles and density of 5.5. Thus, 1 G.

A planet the same size, but made of solid iron (density of 7.9) comes to 1.45 G

An Earth sized planet made entirely of water comes to 0.18 G

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fliguana t1_ixwnv90 wrote

>Not actually just the mass.

Gravity field outside the planet is determined by the mass, nothing else.

Field being the key word. 🧐

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haysoos2 t1_ixwpnks wrote

And the strength of that field is determined by your distance from the center of that mass.

Y'all are making this far more complicated than it needs to be. For pretty much all important considerations, the only number you need to know is a planet's surface gravity. This will govern such things as the escape velocity, and anything actually relevant to living or adventuring on that planet, like how far you can throw a grenade, how high you can jump, or how many puppies you can carry.

My formula will give you a close enough measure of that surface gravity for any planet.

Edit: changed "calculation" to "formula", which is a more accurate description

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fliguana t1_ixwq0dy wrote

>For pretty much all important considerations, the only number you need to know is a planet's surface gravity. This will govern such things as the escape velocity

Incorrect. Two planets with identical surface gravity can have different escape velocities.

Simplification is good to a point, dumbing down leads to errors.

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haysoos2 t1_ixwrzrc wrote

Ok, admittedly escape velocity will be a little different. In my examples above the escape velocity on the iron planet will be 1.7 times higher than on Earth, and on the water planet it will only be about 20% that of Earth's escape velocity, but it's close enough that for say, a fiction based on the Star Wars universe it will be much closer than anything actually used in the series.

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