Three blocks with masses m, 2m and 3m are connected by strings, as shown in the figure. After an upward force F is applied on block m, the masses move upward at constant speed υ. What is the net force on the block of mass 2m? (g is the acceleration due to gravity)

Let T1 be tension in string connecting m and 2m and T2 be tension in string connecting 2m and 3m.            Let a be common acceleration of the system.∴  a=F−m+2m+3mgm+2m+3m=F−6m6mAs the system moves with constant speed, therefore, a = 0∴   F−6mg=0  or  F=6mgThe free body diagram of block m is as shown in the figure.The equation of motion of block of mass m isF−T1−mg=0 6mg−T1−mg=0 T1=5mg        −−−(i)The free body diagram of block of mass 2m is as shown in the figure.The equation of motion of block of mass 2m isT1−T2−2mg=0 5mg−T2−2mg=0     Using  i T2=3mgThe free body diagram of block of mass3m is as shown in the figure.The equation of motion of block of mass 3m isT2−3mg=0 T2=3mgNet force on the block of mass 2m isFnet=T1−T2−2mg =5mg−3mg−2mg=0Alternate solutionAs all blocks are moving with constant speed, therefore, acceleration is zero. So net force on each block is zero.