The motion of a leaking oscillator -- a study for the physics class
  This work is basically about the general form of Newton's second law for variable mass problems. We develop a model for describing the motion of the one-dimensional oscillator with a variable mass within the framework of classroom physics. We present a simple numerical procedure for the solution of the equation of motion of the system to be implemented by students and teachers. Interesting qualitative concepts as well as quantitative results for the focused problem are presented.   The topic has pedagogical value both from theoretical and experimental point of view. However, this article considers only theoretical aspects of the problem. The work is addressed to basic physics courses at undergraduate level. 
Hilario Rodrigues, Nelson Panza, Dirceu Portes Jr, and Alexandre   Soares
physics.ed-ph