I believe a way to view it is as such 1) coffee is always under constant acceleration (g). 2) constant acceleration (say in the X direction instead of Y) woul

I believe a way to view it is as such 1) coffee is always under constant acceleration (g). 2) constant acceleration (say in the X direction instead of Y) would just mean a constant "tilt" within the cup. compare this to an airplane that is not "accelerating" and just at a constant X velocity, coffee would look "still/flat" in your cup. 3) its only the jerk that changes the "tilt" within the cup (and hence causes the spill)