Dear guys, Recently, i am confused with a problem in my textbook of mechanics. The question is, suppose there is a disk, placed horizontally, rotate about its center with angular velocity ω. A ball move with respect to the center of the disk in a trace of Archimedean spiral r=αθ. The angular momentum of the ball is conserved with respect to the center O. What is the real force Fθ and Fr acting on that ball ? this is the question and the solution given in the textbook, since they are written in chinese, i translate it into english Here is my question. From the solution we can see that the θ is the angular in the disk reference system. And they use the statement that the angular momentum is constant. i dont know which reference system should this statement be applied, whether the disk-reference system which is non-inertial or the laboratory frame of reference which is inertial? For me, i think it should be the laboratory frame of reference. and because the disk-reference system is non-inertial, so the angular momentum with respect to the disk-reference system is no longer conserved so it should not be constant anymore sorry for the long question, i hope i explained both the question and my opinion well. thanks for your help!