In figure m1=1kg, m2=2kg. Pulleys are massless and strings are light. For what value of M the mass m1 moves with constant velocity neglecting friction?

For the Mass m1 to move in constant velocity the acceleration of m1 should be zero, for the the tension in the string joining m1& m2 should be 10N.acceleration of pulley(a3) = a1+ a2/2 = 10/4 (a1=0)2x10 =M x a3 Hence, M=8kg