Hi, I would like to understand what is the difference between the case when w=0 and the wase when w>0 in this device: http://imageshack.com/a/img540/3493/Ntztkk.png [Broken] The arm is turning clockwise at w. The hydraulic pump is fixed to the arm. A disk at the end of the arm can turn around itself and it turns at start at w too. The shape of the disk is like a turbine. The hydraulic fluid gives a force F1 to the disk. F1 is transmitted to the arm: F3, and the reaction of the arm is F2 to the disk. The disk receives a torque Fr with 'r' the radius of the disk. |F1|=|F2|=|F3|=|F|. The arm receives the torque FR. The fluid is recovered by the pump, the fluid is in closed circuit. An external device not drawn recover energy from the arm for keep constant the rotationnal velocity of the arm. So I guess w constant for the arm (not for the disk). I can understand the sum of energy when w=0 but when w>0 and especially if 'w' is very high it seems te arm receives the torque FR and the work from the arm is FRwt. So the pump must give more energy when w>0 than w=0 but how and why ?