A Simple and Effective Magnetic Dynamometer to Teach Newton's Third Law

Newton's laws of mechanics are a cornerstone in the development of modern science and its learning can be the very first opportunity for students to defy their common sense perceptions and replace them with a more robust world view. However, despite their fundamental importance, many studies demonstrate how difficult is the true comprehension of the basic aspects of Newton's laws. Therefore, any effort to increase the possibilities of teaching them is welcome. It is in this sense that we propose an experimental apparatus to assist teachers in the specific case of the Newton's action-reaction third law. Our system is formed by a pair of dynamometers made from a flexible plastic strip to where small neodymium magnetic disks are attached. Forces are exchanged by means of the magnetic fields and the plastic strip deflection angle indicates the force magnitudes. This is a very cheap, easy to build and effective arrangement, suitable for numerous classes that demand many experimental kits.