Calculating a planet's mass & gravitational pull

[Baron Munchausen]

Originally posted by dogscoff:

Tomorrow I'm going to have a go at putting all of this functionality into java so I can offer it on my website.
Planned features:
-flexible input values (ie it can automatically calculate circumference from diameter or vice versa.)
-Side by side comparison with a selectable "benchmark" planet (Earth, Mars, Jupiter, Plenty... any others I should have?
-Choose between raw data comparison or proportional comparison (ie Planet X has 125% earth's land surface area)
-Variable input units (miles or kilometres etc)

Anyone likely to use this? I doubt it will see a lot of usage, but thought it might be a handy toy for sci-fi writers. Any particular features I ought to implement? Even if no-one wants it I'll do it anyway as a programming excercise for myself.[/QB]

Aaron Hall? You ought to share it with him. It might improve SE V.

[Suicide Junkie]

The stuff above you will cancel out with the stuff at the same height on the other sides of the planet.
The stuff above your horizon pulls you away, while the stuff below your horizon at the same altitude pulls you towards (weaker due to distance, but more mass) Given reasonably constant density from one side to the other, it cancels out.

Stormy weather would make for small differences.
Sunlight heating one side would make for bigger differences.
But the density can't vary by that much, since the high pressure air will just blow into the low pressure zone and equalize it.

[Slick]

The kg-1 and s-2 refer to exponents in this limited UBB system. Negative means in the denominator, so the units would be: m^3 / [kg x s^2]

Ummm... I didn't understand any of that. Sorry, can you put it into language that can be understood by someone of my high mean density?

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I appologize for being hard to understand. The limitations of this UBB make it hard to use normal mathematical notation.

The units of the gravitational constant G are:

code:

---

                    3

                   m

           ------------------

                       2

               (kg) (s) 

---

Hope this helps.

Slick.

[Fyron]

quote:
--------------------------------------------------------------------------------

Where gravitational constant G = 6.67300 × 10-11 m3 kg-1 s-2

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Glurp! so if my distant maths lessons are correct, that would be -6.67300 with the decimal place moved 11 spaces to the right. Correct? What does the kg-1 s-2 mean?

Not quite. It is not negative, just really close to 0. A negative exponent does not change the sign of the base number.

[jimbob]

right, so 6.67300 × 10-11 m3 kg-1 s-2 could be written as:

6.673 x 10^-11 m^3/kg*s^2

or

0.00000000006673 m^3/kg*s^2

I like to put the breaks in every 1000 so I usually write this:

0.00 000 000 006 673

This, as IF said, is quite different than:

-0.00 000 000 006 673

I hope that this is helpful, and isn't construde as beating a dead horse

[ August 21, 2003, 04:31: Message edited by: jimbob ]

[dogscoff]

Thanks everyone, I think I have the spreadsheet working now. For Earth I get a gravity value of 9840866.19 (starting with a diameter of 12756km and density of 5520kg/m^3). For Mars I get 3736591.2 (diameter 6794.4, density 3935). That's exactly right in proportion to one another, but I have no idea whether or not the values themselves are accurate- all the websites that list such data measure surface gravity in comparison to Earth's.
Also, I haven't factored in rotation which (apparently) increases the effective gravitational pull.

I'm still a little fuzzy on the kg-1 s-2, although I now understand that it's just the unit by which gravity is measured. How would you pronounce it? What does that s stand for? Is it seconds?

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Any O&C fans in the audience (if I haven't killed them all off with the lack of updates) may be interested to know that I've now used the spreadsheet to come up with some values for Ceres IV (Plenty).
diameter: ~19000km.
Circumference: ~60,000 km.
Surface area: 2.2 times that of Earth, but I want at least 85 or 90% of it to be water, which leaves betwee 1.15 and 0.77 times Earth's land area. I'll probably go for 90%.
Density: Not sure yet. With an Earth-like density, it would be about 1.5 times Earth gravity (bloody hard work, but probably livable). Interestingly enough, with a Mars-like density, the number comes out only a little above Earth's gravity.

I always intended it to be high in mins and orgs, but very low in rads, so that would fit in well with low density and therefore only-slightly-higher-than-Earth gravity, and it would also help to explain (along with all the beaches) why the place is so damned popular.

Of course, all these numbers are muddied in the text by literary vagueness, so none of them have to be particularly precise.

[Asmala]

Originally posted by dogscoff:
For Earth I get a gravity value of 9840866.19 (starting with a diameter of 12756km and density of 5520kg/m^3).

Did you remember convert the diameter to meters? The answer (9840866.19) is right, but the unit would be μm/s^2.

I'm still a little fuzzy on the kg-1 s-2, although I now understand that it's just the unit by which gravity is measured. How would you pronounce it? What does that s stand for? Is it seconds?

Yes s is seconds. s-2 is equal to s^-2 which is equal to 1/s^2