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
Reduced-gravity aircraft Reduced-gravity aircraft A reduced-gravity aircraft is a type of fixed-wing aircraft that provides brief near-weightless environments for training astronauts, conducting research and making gravity-free movie shots. Versions of such airplanes were operated by the NASA Reduced Gravity Research Program. The unofficial nickname "vomit comet" became popular among those who experienced their operation. Parabolic flight as a way of simulating weightlessness was first proposed by the German aerospace engineer Fritz Haber and the German physicist Heinz Haber in 1950. Both had been brought to the US after World War II as part of Operation Paperclip. Parabolic flights are also used to