Meriam, Kraige & Bolton — Kinetics of Systems of Particles: Problem 4_67

4/67 A railroad coal car has an empty mass of 25 Mg and carries a total load of 90 Mg of coal. The bins are equipped with bottom doors which permit discharging coal through an opening between the rails. If the car dumps coal at the rate of 10 Mg/s in a downward direction relative to the car, and if frictional resistance to motion is 20 N per megagram of total remaining mass, determine the coupler force P required to give the car an acceleration of 0.045 m/s² in the direction of P at the instant when half the coal has been dumped.