Hi, I'm interesting about the torque in the system like I drawn. I drawn only additional forces from curve, not the true forces from absolute pressure. The shape is full with a lot of very small circular balls (blue color). A pressure is apply from external system. The goal is to have FR perpendiculary to the movement, for that the pressure in the bottom system must be higher to the upper part. This difference of pressure is in function of sin(x). If FR is perpendiculary to the movement, it cannot give a torque and the force FR can be cancel by mechanical system. In the center of rotation (red point), -F1 and -F2 can be cancel without need a torque because the radius is zero. The sum of torque come from forces from up and down curves, like the radius is in function of x and sin(x) I don't understand how the sum of torque can be zero ? Tell me if it's not clear Edit: for example with length = 100 and radius = 1, the torque from a local force at bottom is like 1/sin(1/100) = 100.0016 but at top it is like 1*(100-1)=99, if the calculation is done for each point, the torque at bottom is greater than at top.