(j) What approximations or simplifying assumptions did you have to make in your analysis which might not be adequately valid? (Don't check any approximations or simplifying assumptions which in fact have negligible effects on your numerical results.) If the momentum weren't changing, the momentum would remain zero forever. Since the net force must be zero when the momentum is zero, and since dpy/dt is equal to the net force, dpy/dt must be zero. Since the momentum is zero, the momentum isn't changing. After a very short time the momentum will be upward (and nonzero). A very short time ago the momentum was downward (and nonzero). Check to make sure that the magnitudes of the velocity and force vectors shown in your diagram number 4 are consistent with your analysis of parts (c), (d), and (e)