If one assumes that a projectile encounters an air resistance
proportional to velocity, one can write a differential equation like
mr''=-mgj - kr' which can be solved for r using an integrating factor
e^(kt/m).
The final solution for r, however, does not admit an interpretation for
k=0. Why is it that one does not get the ideal case to come from this
more general case when k=0?
I hope that the question makes sense. A student pointed this out and I
was unable to give any very good response.