oye look.........first of all angular momentum is conserved abt the pt of contact..........ill tell u how..................

all forces comming into picture.....the infinite normal force.......the infinite frictional force........are acting at the pt of contact.....so these forces will not produce any torque abt the pt of contact as the moment arm(or watevr u call it) length =0...............hence the net external torque abt that pt =0.............nudge me if u have further doubts.........

as the disc is coming to hit the plane ull have a normal force acting on the disc at the pt of contact with the ground in such a way that it counter balances the effect of mg as a rotational cause.........

Let me put it clear....im conserving "angular" momentum abt the pt of contact with the incline plane becoz there is no external torque abt the pt of contact with the incline plane............

oye sarthak wat im trying to explain to u is that there is a normal force provided by the ground to the disc rite......ull also agree that this normal force provides a torque abt the pt of contact.........

the normal force provided is so adjusted that the torque due to mg is balanced by the torque due to this force.........

so on the net ther's no external torque abt the pt of contact........thats my openion........

k......ive posted the soln. to this one........

any one having further doubts can approach my nudge book.........

but i have seen some older editions of  mtg  physics for you magazines.............

the answer  given is   3 v^2 / 4g  as i have  mentioned earlier.......

and i have not understood what u have given in the above post...........

please specify  the answer and explain how u have got it...................

can u also tell  me from which book u have referred this concept.........................please.................

i dont want to argue with u but i wont agree with ur previous posts..........................

please specify the book...............................................