http://www.youtube.com/watch?v=zLy0IQT8ssk#t=10m50s objects with different radius got same speed. Lecturer says it's because this is independent from radius and mass. But the hollow one is slower. Lecturer say it's because of moment of interia. But moment of interia is radius dependent. So in example with different radiuses moment of interia is also different. So why speed is the same?