I have a question which puzzles me since the day my tutor pointed out to me. A uniform solid disk is set into rotation with an angular speed wi about an axis through its center. While still rotating at this speed, the disk is placed into contact with a horizontal surface and released as shown in the figure below. (a) What is the angular speed of the disk once pure rolling takes place? (Hint: Consider the torques about the centre of mass.) [Ans: wi/3 ] (b) Find the fractional loss in kinetic energy from the time the disk is released until pure rolling occurs. [Ans: −2/3 ] I did the first part by using the method of conservation of angular momentum which involve the change in moment of inertia and I got the exact answer. However, my tutor said that my concept is wrong even though my answer is correct. After getting the first part, we can easily work out the second part. The thing is that I don't know why my concept is wrong in the first part. Can somebody please explain it to me.