  Save perhaps for the attribution of the F=ma form of the second law to the Principia, the most widespread mistake about Newton's three laws of motion is that they alone sufficed for all problems in classical mechanics. Those who developed what we now call Newtonian mechanics during the eighteenth century at all times appreciated how far from the truth this is. Newton's three laws of motion suffice for problems involving what Euler dubbed “point-masses.” Indeed, once given the forces acting on a point mass, the three laws hold for all point-masses, including those that lie within bodies. But the three laws must be supplemented by further principles for a whole host of celebrated problems involving bodies, rigid or otherwise, that are not mere point-masses. Perhaps the simplest prominent example at the time was the problem of a small arc circular pendulum with two (or more) point-mass bobs along the string. Huygens had solved this problem in the part of his Horologium Oscillatorium entitled “The Center of Oscillation,” in the process providing the theoretical basis for using added masses to tune pendulum  clocks.[28] The reason why Newton's three laws of motion have to be supplemented to solve this problem is easy to see. Consider the case of a pendulum with two point-masses along the length of a rigid string. The outer point-mass has the effect of reducing the speed of the inner one, versus what it would have had without the outer one, and the inner point-mass increases the speed of the outer one. In other words, motion is transferred from the inner one to the outer one along the segment of the string joining them. Once the force transmitted to each point-mass along the string is known, Newton's three laws of motion are sufficient to determine the motion. But his three laws are not sufficient to determine what this force transmitted along the string is. Some other principle beyond them is needed to solve the problem. Which principle is to be preferred in solving this problem became a celebrated issue extending across most of the eighteenth  century.[29]