relative to the rotating frame of reference: Next we need to imagine
that the carousel is the Earth and the two ball tossers are on the
earth, and rotate with it. The ball tossers observe the ball's motion
relative to the carousel and also observe the "inertial"
frame rotating the other way. You experience this when you are in
a moving car. You could take the point of view that everything in
the car is be fixed, but the outside terrain is moving past you. Alternatively,
you could take the point of view that the outside terrain is fixed
and the car, and everything in it, moves. If you didn't know better,
you couldn't tell whether the car was moving forward, or the outside
terrain was moving backward.