1. The problem statement, all variables and given/known data The end of a chain of length L and mass per unit length ρ, which is piled up on a horizontal platform is lifted vertically with a constant velocity u by a variable force F. Find F as a function of height x of the end above platform. A)ρ(gx+2u^2) B)ρ(gx+u^2) C)ρ(2gx+ρu^2) D)ρ(u^2-gx) 2. Relevant equations 3. The attempt at a solution The chain goes up with a constant speed so at any instant the net force should be zero i.e. F=mg. If the height of the end is x, then m=ρx therefore F=ρgx. But there is no such option. Any help is appreciated. Thanks!