well, to plot in mathematica you need to set max and min values for the x axis. I cant factorise it so i dont know what to enter as the x values, and i tried i entering some x values like: -10, 20 and 10, 100, but i always get an error that it cant be graphed.
Any idea on what i can set x axis to graph this beast?
I don't know why you'd get an error. There should be no problem graphing it (you do realise there's a vertical asymptote at r = 0 .....?). Have you tried using a humble graphics calculator if you're having trouble graphing it with Mathematica? Setting useful windows is often trial and error if you have little idea what the graph looks like. It can be more an art than a science.
No, i had no idea there is an asymptote at x=0, so i tried graphing it from x=-10 to x=-1 (to avoid the asymptote if thats whats causing the problem with mathematica), but it still doesnt work. I found the explanation of the error, it says "Error messages are generated in this example because the symbolic parameters in the function to be plotted do not have numerical values."
So there can not be numerical values for the graph, i dont know how to get around that. I dont have a graphics calculator and im a scientist (chemist) maybe thats why im so bad at these questions...
I don't know why you're getting an error. Perhaps you need to check the documentation on how to plot a graph in Mathematica.
Perhaps, as CaptainB suggested, there's a restriction that r > 0? Since U = U(r) is probably modelling something like the Van der Waals potential energy (you're the chemist, put your thinking cap on).