Ugo Besson

Work and energy in the presence of friction: the need for a mesoscopic analysis

This paper deals with some of the difficulties involved in correctly linking the mechanical and thermodynamic descriptions of situations where there are bodies in motion and changes in temperature and/or heat exchanges. It proposes and discusses a definition of internal energy and shows that the total work of the non-conservative forces, such as frictional forces, always equals zero. The energy balances of two bodies in motion in the presence of sliding friction are analysed in this way; the calculation of the work of frictional forces is discussed, taking into account the mesoscopic level.

Some Features of Causal Reasoning: Common Sense and Physics Teaching.

The idea of causality is central in science and has long given rise to debate among philosophers and scientists. While the tendency to avoid causality seems to have become dominant in science and philosophy, research in science education has shown the strong presence in common reasoning of causal explanations, often conceived as a ‘mechanism’ capable of accounting for physical transformations. Some researchers have proposed using this common causal reasoning as a basis for teaching–learning sequences, especially in electricity and mechanics. This paper analyses some features of causal reasoning used in physics by students, using questionnaires and interviews involving students and teachers. This study has shown three aspects which are related to one another: a confusion between efficient and contingent causes, between the conditions of occurrence of a phenomenon and the cause actually producing it; a tendency to ‘displace’ causes, skipping intermediate objects; and a difficulty in connecting local causes and global effects. The paper highlights the differences between common reasoning and scientific usage, and their effect on learning. In fact, these trends of reasoning must be taken into account in teaching: they should be considered not only as creating an obstacle to learning physics, but also as resources at the learner’s disposal.

The distinction between heat and work: an approach based on a classical mechanical model

The distinction between work and heat is obvious in most typical situations, but becomes difficult in certain critical cases. The subject is discussed in texts on thermodynamics and has long given rise to debate. This paper presents an approach based on a mesoscopic analysis, using a simple mechanical model, in which bodies are made up of particles (representing atoms and/or molecules) treated as material points interacting with forces that obey Newtons laws. The sum of the work done by these microscopic forces is split into two terms representing the macroscopic quantities work and heat.

A Mesoscopic Model of Liquids for Teaching Fluid Statics

After a study on students’ conceptions concerning pressure in fluids, we elaborated a short teaching sequence on the statics of liquids, implemented and evaluated for two years among science students in the first university year. The sequence makes use of a model of liquids at the mesoscopic level through an analogy with a set of sponge balls, treated first as mechanical entities, then as a model of elements of fluids. The aims and the results of this experimentation are presented and discussed.

How does weight depend on mountain altitude

Using elementary mechanics and simplified models, this paper studies how the weight of an object varies with mountain altitude. After a brief discussion of the role of centrifugal forces, the effects of the mass of the mountain above and below the reference point are discussed and calculated, as well as the effects of the Earths curvature. The mountain shape factor is defined and calculated as a function of an appropriate shape parameter for a few cases of regular geometric solids. We discuss the theoretical conditions under which the weight could increase with increasing mountain elevation.