1. The problem statement, all variables and given/known data A railroad car moves under a grain elevator at a constant speed of 4.50 m/s. Grain drops into the car at the rate of 420 kg/min. What is the magnitude of the force needed to keep the car moving at constant speed if friction is negligible? 2. Relevant equations U=V+dV-Vrel , U is the velocity of dM, which is 0 in this case dM/dt • Vrel = M • dV/dt where V is the velocity not volume 3. The attempt at a solution So from the question, dM/dt is 420kg/min=70kg/s, I played with the 1st rocket equation, was able to get (4.5+dV)*70=M*a=Force needed. But how do I find dV with limited information on the system given?