Hi guys: I have 3 questions: Imagine an open differential that looks like a gear ring : and now let's call the rings (orbital gear) the wheel gear, the gear in the center the center gear. 1. is the reason why torque from input shaft is evenly split to both wheels regardless of different speed because : As long as the speed difference is constant, the differential has a constant angular velocity and hence net torque must equal to zero? 2. In the gear ring video, if i am to apply a torque not from the differential(center gear in track/sun gear) but now either : hold one of the wheel ring(orbital gear) fixed and turn the other one, or if I turn both of them at the opposite direction. will it be correct to say that under no friction, there will only be forces between the wheel ring and track gear be non-zero during acceleration? in other word if i continue to exert a force from one wheel ring I will be accelerating the angular speed and speed of the center gear? and just the angular acceleration if i turn both wheel ring together? 3. Finally, a more fundamental question: why is horse power equal to force times velocity (or T omega)? is it because we first defined kinetic energy to be 1/2mv^2? What prompted the people in the old times to define energy in such a way? thanks!