Resumen de Waterways and Rocket-Propulsion Problems
Miguel Lagos, Mario I. Molina, Rodrigo Paredes, Milton Elgueta
Here, we explore conservation of momentum for variable-mass systems by means of two systems: a stalled boat in a lagoon and the propulsion of a rocket. The rocket propulsion problem is shown to be equivalent to the stalled boat problem. Both examples are solved using the concept of momentum conservation and using just elementary though refined mathematical methods. A physical concept that can be difficult for a physics student to grasp is the idea of a conservation law. In the teaching of mechanics, many efforts are directed toward the learning and application of Newton’s laws. However, the related concepts of momentum and energy conservation are often given less attention. Yet, students are sometimes able to recognize the consequences of conservation of momentum in many experiences of everyday life, especially if they are called out explicitly and intentionally. In this short article, we propose a couple of physical problems whose solution rests on an application of the conservation of momentum, using elementary methods. The complete solutions follow with no use of differential analysis. However, inserted in them lies the mathematical concept of limit, which allows one going from the discrete (stalled boat in a lagoon) description to the continuous one (propulsion of a rocket).
