What Force and Torque Are Working with the Roberval Balance

The Roberval balance mentioned by E. Scott Barr presents a phenomenon against intuition that appears to violate the principle of torque equilibrium. Roberval called his balance a “static enigma.” This apparent paradox went unexplained for well over a century until L. Poinsot in 1804 explained that the thrusts created in the parallelogram canceled each other out, leaving only the downward force of the suspended thrusts. Others also attempted to explain the Roberval balance, specifically H. Richard Crane and Paul Chagnon, and a partial analysis was conducted with two exercises by E. J. Routh. Crane focused on the change of an object’s height; when tilted, there is no change in the potential energy even when the location of the object is changed. Chagnon noted that even when the location of the object is changed, the vertical force acting on balance remains constant. Despite the fact that these researchers attempted to explain the Roberval balance successfully in several other ways, their works all pertained...