1. The problem statement, all variables and given/known data Find the period of a pendulum consisting of a disk of mass M and radius R fixed to the end of the rod of length l and mass m. How does the period change if the disk is mounted to the rod by a friction less bearing so that it is perfectly free to spin? The centre of the disk is attached to the rod. 3. The attempt at a solution I can find the position of the centre of mass of the system, the torque due to gravity and the moment of inertia of the system about the pivot. From this quantities i shall form the equation of motion and thus i can find the time period. Now i see no reason why the period will change if the disk is free to spin. Firstly i think the disk will not spin since there is no torque acting on it. Both gravity and the force exerted by the rod pass through the bearing. If it also spins the position of centre of mass doesn't change. The moment of inertia of the system too remains unchanged. Am I correct since I am feeling that I missed something?