The problem involved calculating the path of a projectile that is subject to both gravity and air resistance. Ray also solved a second problem, involving a body colliding with a wall, as part of a school project.
That makes sense. So... add air resistance to kinematic equations [easier said than done]... I can see that.
The second one... object colliding with a wall. An exact analytic solution would be nice. Although it probably includes a number of simplifying assumptions. Is the ball compressible [like a spring]? Does this only work for balls, or other objects as well?
Apparently the solution was already implicitly known [but still impressive if independently determined]: http://www.df.uba.ar...mechan/air0.pdf
As for the rest... it's in this picture, but hard to see... and in German...