Consider the simplest, bleakest situation imaginable: an isolated body alone in an empty universe. How would it move? Well, since there’s nothing around to push it or pull it, the force on the body is zero: F = 0. Then, since m is not zero (assuming the body has some mass), Newton’s law yields F/m = a = 0, which implies that dv/dt = 0 as well. But dv/dt =0 means the lonesome body’s velocity doesn’t change during the infinitesimal time interval dt. Nor does it change during the next interval, or the one after that. The upshot is that when F = 0, a body maintains its velocity forever. This is
...more
