Talk:Roche limit/Archive 1
Great article, I am very impressed with all of the stuff on the planets and tidal forces. I esp. liked the part about some moons being within the Roche limit but one one hand they are held together by tensil strength so they do not break apart but on the other any object on the surface not tied down might leave the surface.--ShaunMacPherson 23:02, 18 Mar 2004 (UTC)
Yes, I have to say this is one of the coolest articles I have ever read here. I came to the discussion page just to say that. --P3d0 13:51, 18 Aug 2004 (UTC)
A very good article, properly organized to avoid bothering the non-techies with the math. But at the end it reminds me of an old Harris cartoon: Two mathematicians in front of a blackboard full of equations, one of them saying, "I follow your derivation down to the point where it says 'Here a miracle happens.'"
I got rid of one miracle by fixing the typo in an exponent. Then I set to work finishing the derivation: expand terms, simplify, forget s where it's added to d, simplify again. With great embarrassment I confess that every time I did it, I got the wrong answer, by a factor of 2.
The Wolfram site's derivation is much easier, and comes out right, even though one has to supply a derivation for the tidal force formula. It uses a mass element μ on the orbiting object, at distance r from the center. That difference should all cancel out, duhh.
It would be a favor, and would improve the article, if someone who can fill out this derivation would put up a couple of intermediate results to replace the arm-waving. Or I could replace the derivation with one using the other approach, which is not a copyvio, but we'd need a different diagram. Dandrake 17:52, Aug 24, 2004 (UTC)
- I got the start of the derivation from here and I didn't check it :-( I've started working it out in the article and it looks to me like I'm going to be out by a factor of 8! It's very late I cannot see what I'm doing wrong. Check my working please. If we can't sort it out then it's very easy to change the diagram, if going with the Wolfram site's derivation makes things easier. Theresa Knott 00:46, 25 Aug 2004 (UTC)
- Yes, that factor of 8 is the factor of 2 I was talking about (it emds up under the cube root, right?) You have no idea how much better this makes me feel. Dandrake 07:35, Aug 25, 2004 (UTC)
- I think the problem is the part where you define what the tidal force is. Instead of using the difference between the gravity of the two bodies, you should start off with the formula from Tidal_force (and possibly link to that page) Wuzzeb 01:59, 25 Aug 2004 (UTC)
- Ok I fixed it, but it is still technically wrong. The tidal force is from a gradient of the force across one body, and splitting the body up into two (equal) pieces is the wrong way to aproach the problem. Wuzzeb 02:43, 25 Aug 2004 (UTC)