As
M1>M2 , so tension in thread connecting the block
B and support
C is
(M1−M2)g. When this thread is burnt, this tension disappears. In this case tension in spring is
M1g and it is elongated. So, tension in stringconnecting
A and
B is
M1g. Hence, resultant force on
A remains zero because tension in string is balanced by tension in spring. The block
B has a net upward force
(M1−M2)g.
so initial acceleration of block
B is
Thus, initial acceleration of
M1 is zero and that of
M2 is
upward.