Consider two stationary blocks of same mass suspended by a massless string over some kind of pulley. The net force on both of them is clearly 0. Now suppose we somehow gave on of them a downwards velocity by some kind of energy transfer. Then intuitively the block would start to fall down. But is that in agreement with Fres = ma? Because the net force on both blocks would be zero and thus allow no acceleration. Though of course Fres = ma does allow for a constant velocity. Therefore: Would the block just fall down with same velocity forever?