Why doesn't the ISS (International Space Station) spin to imitate Earth gravity?
I remember reading somewhere that centrifugal force isn't enough to help people avoid things like osteoporosis, etc., but I can't imagine why not. I have thought about this with all of my brains and still don't get it. It seems to me that it's doable -- you'd have to have like a ball-bearing docking center (or whatever -- we'll work on that later) so folks could get on/off, but assuming that problem's solved, what are the other problems? Getting the spin started might take extra weight/fuel, but once it starts spinning won't it spin forever? I'm not mathy so consider NOT citing advanced physics in your answer, but I'm picturing like a[t least a] two-armed ship where the dock is the center (i.e., no "gravity") and is stationary (on spaceship-equivalent ball bearings) and the arms get progressively more gravityish until you get to the tips, wherein are the work/living areas. Tips are bulbous or otherwise larger than the arms, which are long and...arm-y.
Can't we make this happen?
EDIT: I shouldn't have specifically mentioned the ISS since my question was just as much about a theoretical spinning ship's effects on its inhabitants. In putting this question here (in Worldbuilding), I was imagining a ship or structure, in space, meant for long-term living. I'm new on StackExchange, too, and am therefore an idiot w/regard to posting in wrong areas. :)
I did search all over the place for a similar question/answers, but didn't find one (was probably not doing the search right). I'm sorry if I goofed the placement.