listen....
it'd be fine if this was an introduction to newtonian mechanics in a grade 12 class. but, he just spent three hours building up an extremely abstract meta theory, to throw it all away in a bunch of backwards assumptions that should not be made in the context of doing this in such an abstract way.
the earth is not flat, and dU/dx is consequently not zero.
yes, this is a course in classical mechanics. and, yes, that's a simplifying assumption that is just fine in the context of classical mechanics. but, wasn't the point of building up the theory he just built up to work through those assumptions more rigorously? why bother if you're just going to make naive newtonian errors?
there should be a limiting process, there - and, yes, you're going to end up with zero in the limit in the end. but i came here to see that done.
for now, i'm just going to ignore this catastrophe and otherwise let him finish.
if you're keeping track, though, we've learned watching these lectures that susskind is both a conspiracy theorist and a flat-earther.