forces apparently arm chair physicists argue over whether or not the centrifugal force is real or fictional. fooey. it is and it isn't. it depends on what reference frame you choose. i explain. tie bob to a string. and swing him around in a circle. the centrifugal force is directed away from the center. it's center fleeing. which is a nice mnemonic cause fleeing and fugal alliterate nicely. the centripetal force is directed towards the center. traditionally, it's center seeking. which pseudo rhymes with fleeing. i'm not very traditional so i tend to think of it as center peeing. which is a superior mnemonic cause peeing and petal alliterate nicely. and peeing and fleeing are true rhymes. bonusly, one pees towards the center of the toilet. one does not pee away from the toilet. at least, not in my house. so anywho, bob is revolving around the toilet. in the inertial reference frame, bob is accelerating towards the toilet. the center peeing force is provided by the tension in the string. the sum of the forces equals mass times acceleration. specifically mass * angular velocity squared * distance to toilet. and everything is good. standard first year high school physics. the other way to look at things is to use a non-inertial rotating reference frame. in this case, bob is stationary relative to the reference frame. bob is in equilibrium. therefore the sum of the force vectors must be zero. therefore, the center fleeing force is equal but opposite to the center peeing force. specifically it is mass * angular velocity squared * distance to toilet. which should sound awful familiar. in fact, the only difference between the inertial and rotating reference frames is the side of the equation this term lives on. call it a force and put in on the sum of the forces side. call it an acceleration and put it on the acceleration side. it really doesn't matter. at least not math-wise. definition-wise surely. is bob at equilibrium? ie are the sum of the forces zero? heh. trick question. yes (in the rotating non-inertial reference frame) and no (in the intertial reference frame).  ¶ 8:00 AM