Work–energy theorem and friction forces: two experiments

Abstract

Several studies have showed the subsistence, even in students enrolled in scientific degree courses, of spontaneous ideas regarding the motion of bodies that conflict with Newton’s laws. One of the causes is related to the intuitive preconceptions that students have about the role of friction as a force. In fact, in real world novices do not recognise friction as a force, and for this reason they may believe that a motion with a constant speed needs as a necessary condition the presence of a constant force in the same direction of the motion. In order to face these ‘intuitive ways of reasoning’, in this paper we propose two sequential experiments that can allow undergraduate students to clarify the role of friction forces through the use of the work–energy theorem. This is a necessary first step on the way to a deeper understanding of Newton’s second law. We have planned our experiments in order to strongly reduce quantitative difficult calculations and to facilitate qualitative comprehension of observed phenomena. Moreover, the proposed activities represent two examples of the recurring methodology used in experimental practices, since they offer the possibility to measure very small physical quantities in an indirect way with a higher accuracy than the direct measurements of the same quantities.

Tracker-assisted modelling: simultaneous validation of the conservation law of energy and the work-energy theorem

“…Using the experimental value of g (=9.90 m s 2 ), initial velocity (=−1.21 m s −1 ), mass of the object (=0.031 kg), and the time data, we have computed, the values of the kinetic energy, potential energy, and the total energy using the respective terms in equation (3). That is, the kinetic energy was computed using the first term, potential energy with the second term, and the total energy by summing up the results of the two terms (the same process was used in what follows).…”

“…Then by using the same procedure on the kinetic energy data points (though the absolute values need not be taken) used to obtain the target displacements, the corresponding changes in kinetic energy were obtained. Here the kinetic energies involved were those computed using the first term in the left-hand side of equation (3). The total work done was then plotted against the change in kinetic energy and the resulting plot, along with a linear fit, is presented in figure 7.…”

“…But there is nothing more demonstrative of the correctness of a physical theory or concept other than with an experimental verification. Indeed, experimental demonstrations, for pedagogical purposes, of the conservation law of energy [1,2] and the workenergy theorem [3,4] are well-documented in the literature using varied systems and techniques.…”