irf a couple posts previous was about inertial reference frames. in the case of bob swinging around the toilet, it doesn't really matter if we choose an inertial reference frame or a rotating non-inertial reference frame. let's consider a case where the choice of reference frame does make a difference. bob is sitting on a desk. we automatically draw the force diagram with bob's weight pointing down and an equal but opposite normal force from the desk pointing up. bob's in equilibrium. this is what you'll find in every classroom everywhere in the world throughout time and space. the reference frame of the classroom is so natural and so ubiquitous that no one ever points out that we've made this assumption. and no one ever points out that the classroom reference frame is non-inertial. in the inertial reference frame, the weight (=mg) term moves from the forces side to the acceleration side. mathematically it's equivalent. just like it was when bob was revolving around the toilet. however, it leads to some fractious statements. like bob is not at equilibrium. what!?!? and weight is a fictitious force. wtf?!?! blaspheme! heh. true though. if you're going to say that centrifugal forces are fictitious, then i'm going to say, with equal validity, that weight is also fictitious. it's a matter of choice of reference frame. heck of a way to lose weight. i should market it. i'm gonna be so rich.  ¶ 8:00 AM