Hi friends, Here is a typical problem https://fbcdn-sphotos-d-a.akamaihd.net/hphotos-ak-ash4/432331_2639240957573_1038427614_n.jpg It is an elastic collision. Here I am applying simply the conservation of linear momentum as, Momentum before collision = Momentum after collision i.e. Total translational K.E. of the block will be transferred to the disk. Hence disc's COM will have velocity v after collision. and hence the angular velocity be ω = v/r. But this is not the correct answer. The correct answer is option (b). Please friends help me in this problem. Thank you all in advance.