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 v. What is the net force on the block of mass 2m?

All blocks are moving with constant velocity so net force on all blocks are zero. Since the velocity is constant, the acceleration is zero (a = 0). Newton's second law states that F = ma. With a = 0, the net force on each block must be zero. Therefore, the net force on the block of mass 2m is zero.