what does dx mean?
to mathematicians, dx, dy/dx, etc. are all notation. they don't have a physical meaning,
any more than "+" has a physical meaning. dx represents the fact that we are sampling
at an interval of an infinitesimal. we can't measure dx.
to physicists, there are no infinitesimals. dx is simply a very tiny change in something,
a change so small that we cannot notice it. however, the change is there. that's why
