1. The problem statement, all variables and given/known data [PLAIN]http://img202.imageshack.us/img202/4535/asdty.jpg [Broken] 2. Relevant equations I think the relevant equations are the divergence theorem [URL]http://upload.wikimedia.org/math/0/9/b/09b08625d572f4d7c9ceb7585241dc36.png[/URL] 3. The attempt at a solution Since it hints at a "clever" solution, I assume the difficult part is to find the right path. I think we can take advantage of the divergence theorem to convert the surface integral to a volume integral (well, it's n-dimensional, so from n-1 -> n). I think the flux on opposite sides are canceled out, because the function is a function of radius only? But the velocity can be different on the opposite sides.