Equations of motion modification consistent with conservation of momentum, see variable-mass system. It may be simple to write down the equations of motion in vector form using Newton's laws of motion, but the components may vary in complicated ways with spatial coordinates and time, and solving them is not easy. Often there is an excess of variables to solve for the problem completely, so Newton's laws are not always the most efficient way to determine the motion of a system. In simple cases of rectangular geometry, Newton's laws work fine in Cartesian coordinates, but in other coordinate systems can become dramatically complex. The
Clarke's three laws into any typeface and size from Albertus Extra Bold to Zurich Calligraphic", referring to his memory of "seeing and hearing Linotype machines which slowly converted ‘molten lead into front pages that required two men to lift them’". A fourth law has been proposed for the canon, despite Clarke's declared intention of stopping at three laws. Geoff Holder quotes: "For every expert, there is an equal and opposite expert," which is part of American economist Thomas Sowell's "For every expert, there is an equal and opposite expert, but for every fact there is not necessarily an equal and opposite fact", from