Calculating a planet's mass & gravitational pull

[Suicide Junkie]

Why not just output it in all units simultaneously.
B5 = meters
B7 = B5 * feet/meter
...etc...

[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?