The following came from a discussion about launching a balloon. Suppose you have a heavy coiled rope of total length, L, and constant linear density, MU. You take hold of one end of the rope and pull it vertically up with a force, F(t), so that the tip of the rope moves at a constant velocity, v. What is F(t), assuming the rope is so long that some of it remains coiled on the ground? In drawing our free-body diagram, two questions arise: 1) Can you define our system to be just the vertical length of rope (of length, y(t)) --- see figures 1 & 2? 2) If so, how do we calculate the force that the coiled portion exerts on the vertical section, F(coil on rope)? There must be a force, otherwise the coiled portion wouldn’t unwind. Thanks in advance.