Calculating a planet's mass & gravitational pull

[dogscoff]

That would make it too cluttered. I'm not talking about the spreadsheet any more, I'm currently putting the whole thing into javascript for my webpage.

[dogscoff]

*sigh*

sorry to bother y'all again, but...
Where am I going wrong? I'm trying to work out the calculation needed to derive radius (in km) from a known mass and surface gravity (in metres per second per wotsit):

g=surface gravity, G= gravitational constant, m=mass, r=radius.

g=G*m*(r*1000)^3 - this one works in my excel sheet.
therefore
g/G=m*(r*1000)^3
therefore
(g/G)/m=(r*1000)^3
therefore
cuberoot((g/G)/m)=r*1000
therefore
cuberoot((g/G)/m)/1000=r

Or is that all crud? It's a long time since I studied or used this kind of maths.

Apart from this, the javascript sheet is coming along nicely.

[Suicide Junkie]

Looks good to me

[dogscoff]

Yeah, I thought it looked ok, but all the numbers come out wrong.
Here's the line from the code. Something just has to be wrong with it, but I'll be stuffed if I can figure it out.

cells[radius].value=Math.pow(((cells[gravity].value/GRAV)/cells[mass].value),1/3)/1000;

I've also just put in an alert statement to display the output of the same calculation (in case the number was getting jiggered about elsewhere) but apparently it isn't.

[ August 27, 2003, 14:11: Message edited by: dogscoff ]

[dogscoff]

Hmm, I was *way* off, but now I'm closer. I went back to S_J's original calculation of g = GM/(1000*R)^2
(dunno where I got g=G*m*(r*1000)^3 from)

g = GM/(1000*R)^2
g*(1000*R)^2 = GM
(1000*R)^2 = GM/g
(1000*R) = squareroot(GM/g)
R=(squareroot(GM/g))/1000

In javascript, that comes out as
radius=(Math.sqrt(GRAV*mass)/gravity)/1000

and it works. Sorry for all the fuss, ppl.

[ August 27, 2003, 15:12: Message edited by: dogscoff ]

[Suicide Junkie]

radius=(Math.sqrt(GRAV*mass)/gravity)/1000

Shouldn't you Math.sqrt( grav*mass/gravity ) or somesuch?

[Puke]

GURPS Space (from Steve Jackson Games) used to have work sheets for generating alien planets, that included orbital radius and period, roation, gravity, mass, temerature, etc, etc..

might look for an old copy, or a PDF of the reference sheets Online.