Rotational Mechanics Problem typical one

[thunderhadron]

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.

 

[mfb]

Hence disc's COM will have velocity v after collision. and hence the angular velocity be ω = v/r.

Angular velocity around what?
The surface is rough, so I think you should assume that the disk is rotating on the surface. This needs additional energy, and the disk will be slower than v.