1. The problem statement, all variables and given/known data Two cylinder shaped vessels of cross section A are fixed vertically to a trolley, which was initially at rest. The two vessels are connected with a thin horizontal tube which is equipped with a tap. The distance between the axes of the vessels is L. The vessel in the left hand side is filled with some liquid of density ##\rho##; and at this time the total mass of the system at rest is m. After opening the tap what is the speed of the cart at the moment when the speed of the water levels in the vessels is v? (Rolling friction, friction in the bearings and air-drag are negligible.) (Answer: ##vAL\rho /m##) 2. Relevant equations 3. The attempt at a solution I think I have to use conservation of linear momentum but I am unsure about how to do that. The motion of fluid in the cylinder confuses me. I need a few hints to get started with. Thanks!