While explaining about inertial and non-inertial frame of reference, people give this example-- http://www.phys.unsw.edu.au/einsteinlight/jw/module1_Inertial.htm if you don't wish to follow the link, here is a simple explanation---> there are two person and a rotating disk. Person A is in rotating disk and person B outside. Both will see each other revolving around. But when person A throws a ball towards the rotating person B it won't straight but curving round to meet Person B, but however If person B throws a ball then it goes straight. I think the flaw here is asuming that the force that provides centripetal force to the Person A,(may be frictional force here), is applied to the Person only but not the ball. What if the ball was rolled out on the disk and it had les assume: infinite friction (so that, when the ball is just placed on the side of person A, it won't shoot out of the disk, due to the same friction that holds person A), then the ball will go straight for person A, But curving for person B. Now, can you say Person A is in inertial Frame of reference.