Hi, 1. The problem statement, all variables and given/known data A cable car slides without any friction along a horizontal wire with initial velocity v0. The car is half filled with water and its mass (with the water) is m. At t=0 it starts raining (perpendicular to the ground). The rate at which water is collected in the car is k (=mass/time). At that point the driver opens an aperture at the bottom of the car and water begins leaking so that at any time the amount of water in the car is constant (the amount of water due to rain equals to the amount leaking from the car as it slides). What is the velocity of the car after a time t? 2. Relevant equations 3. The attempt at a solution There is conservation of momentum along the x axis and there are no forces along that axis acting on the car, so why would the velocity change? Why wouldn't v(t)=v0? The solution is dPx/dt = 0 = -kv(t) + m*dv/dt. Well, that should be the equation yielding the solution in any case. I also don't quite understand why dm/dt would be k. Isn't it stated that the amount of water remains constant? Why wouldn't that derivative be equal to zero? I am kindly asking for an explanation.