if3
last post on inertial reference frames. i promise. there's one more case where we naturally choose the reference frame. and that's the case when bob's on a space ship orbiting the earth. we always use the inertial reference frame. and say there are no forces acting on bob. but wait! bob is still affected by gravity. heh. now you're switching to a non-inertial rotating reference frame with gravity. bleh. so now the force diagram has weight pointing down and the centrifugal force pointing up. for every object floating around the spaceship. which horribly clutters the math. and it isn't any more "right" or "wrong" than the inertial frame. which is significantly simpler. and hence, much more useful. which is why bob-on-the-desk and bob-in-spaceship can both be in equilibrium when bob-in-spaceship is floating around the room and bob-on-desk isn't. much to bob-on-desk's disappoint.