Let centre O1 of disc be the origin. Due to symmetry the centre of mass of remaining part will lie at x -axis. Let CM of new disc is at point O2 where, O1O2=αR(given). here mass of cut off portion,m=
π(R)2.M
π(2R)2
=
M
4
and position of its centre of mass, O1O1=R Hence, for remaining part (new disc) xCM=αR=