There is this problem and I do not understand how could this be: A railroad hopper car filled with sand is rolling with an initial speed of 15.0 m/s on straight, horizontal tracks. You can ignore frictional forces on the railroad car. The total mass of the car plus sand is 85,000 kg. The hopper door on the underside of the car is not fully closed, so sand leaks out the bottom of the car and falls to the ground. After 20 minutes, 13,000 kg of sand has leaked out. What is the speed of the railroad car now? The answer is 15 m/s. How could change in mass could not affect speed? initial momentum of the system where sand is about to start leaking: p_i = mv and final momentum: p_f = (m - 13000)v' and solving gives v' = 17.7 m/s, but book gives answer of 15 m/s, what am I missing here? Since most of the sand is on the ground now and do not have speed?