A pulley fixed to the ceiling carries a string with blocks of mass m and 3 m attached to its ends. The masses of string and pulley are negligible. When the system is released, its centre of mass moves with what acceleration
The correct option is B. g/4.
Consider the following diagram,
The acceleration of the whole system can be found by force balance i.e.,
(3m+m)asys=3mg−mg
asys=g2
So, acm=m1a+m2am1+m2=m(g2)+3m(−g2)m+3m
acm=−g4
The negative sign shows that the acceleration is downwards.