[Tutor] Problems using odeint

[Steven D'Aprano]

On Tue, Jan 14, 2014 at 06:40:03PM +0000, Grace Roberts wrote:
[...]
> I've attached a quick print-screen of the code.

I've managed to have a look at the screenshot, there are no obvious 
problems, but I'm not a Scipy expert. If you do get an answer from a 
Scipy mailing list, I would really appreciate it if you were to write 
back here with an explanation.

In the meantime, I have a couple of comments about your code. You 
initialise some constants like this:

G = -6.67*10**-11
Mm = 6.4*10**23

You can write them more compactily as:

G = -6.67e-11
Mm = 6.4e23


Notice that you have made G a negative value? That is unusual, normally 
G is quoted as a positive quantity, G = 6.67e-11 m³/(kg*s²), or the same 
value with units N (m/kg)². Either way, G is given a positive value. 
Possibly making G a negative quantity is causing the problems?


Another minor comment on your code, in the f(x, t) function you have a
line taking a square root:

r = (x[0]**2 + x[2]**2)**0.5


You can improve the accuracy of that radius calculation by using 
the hypot function. First, at the top of the file, import 
the math module:

import math


Then inside the f(x,t) function change the calculation of r to this:

r = math.hypot(x[0], x[2])


Hope this is helpful,



-- 
Steven