Julio Güémez

Toys in physics lectures and demonstrations—a brief review

The use of toys in physics teaching is common. This brief review of the physics of toys intends to show that they are not only very useful in lectures and demonstrations in order to motivate students but also very interesting from a scientific point of view. However, since their physics is sometimes too cumbersome, the effect can be the opposite. We call attention to some subtleties of toys used in physics or in general science teaching.

Experiments with the drinking bird

We present a simple model of the dynamics of the drinking bird and relate its period to the properties of its internal and external liquids. The effect of humidity on the motion is studied and it is shown that there are two evaporation regimes. The results of the model are in agreement with observations.

From mechanics to thermodynamics?analysis of selected examples

We present and discuss a selected set of problems of classical mechanics and thermodynamics. The discussion is based on the use of the impulse-momentum equation simultaneously with the centre-of-mass (pseudo-work) equation or with the first law of thermodynamics, depending on the nature of the problem. Thermodynamical aspects of classical mechanics, namely problems involving non-conservative forces or variation of mechanical energy are discussed, in different reference frames, in connection with the use of one or the other energy equation, and with the compliance of the Principle of Relativity.

Forces on wheels and fuel consumption in cars

Motivated by real classroom discussions, we analyze the forces acting on moving vehicles, specifically friction on their wheels. In typical front-wheel-drive cars when the car accelerates these forces are in the forward direction in the front wheels, but they are in the opposite direction in the rear wheels. The situation may be intriguing for students, but it may also be helpful and stimulating to clarify the role of friction forces on rolling objects. In this paper we also study the thermodynamical aspects of an accelerating car, relating the distance traveled to the amount of fuel consumed. The fuel consumption is explicitly shown to be Galilean invariant and we identify the Gibbs free energy as the relevant quantity that enters into the thermodynamical description of the accelerating car. The more realistic case of the cars motion with the dragging forces taken into account is also discussed.

The Cartesian diver and the fold catastrophe

The motion of the Cartesian diver is studied, both theoretically and experimentally, and interpreted as an example of a fold catastrophe, where the control parameter is the external pressure.

Revisiting Black’s Experiments on the Latent Heats of Water

Historical experiments may help students to better understand some physical phenomena. We reproduced Black’s original experiments on the latent heats of water (fusion and vaporization). To obtain both latent heats with reasonable accuracy we needed concepts, which were not used by Black, such as the water equivalent of a calorimeter and Newton’s law of cooling. The melting experiment is adequate to obtain an accurate value for the latent heat with a small uncertainty, but the same is not true for the vaporization experiment.

An undergraduate exercise in the first law of relativistic thermodynamics

The isothermal compression of an ideal gas is analysed using a relativistic thermodynamics formalism based on the principle of inertia of energy (Einsteins equation) and the asynchronous formulation (Cavalleri and Salgarelli 1969 Nuovo Cimento 42 722–54), which is similar to the formalism developed by van Kampen (1968 Phys. Rev. 173 295–301) and Hamity (1969 Phys. Rev. 187 1745–52). In this 4-vector Minkowski formalism mechanical and thermodynamical processes are described by the first law of thermodynamics expressed as ΔUμ = Wμ + Qμ, in a Lorentz covariant way. This exercise is considered useful for undergraduate physics students interested in foundations of physics, with the only prerequisites in first courses in thermodynamics and special relativity.

Thermodynamics in rotating systems—analysis of selected examples

We solve a set of selected exercises on rotational motion requiring a mechanical and thermodynamical analysis. When non-conservative forces or thermal effects are present, a complete study must use the first law of thermodynamics together with Newton?s second law. The latter is here better expressed in terms of an ?angular? impulse?momentum equation (Poinsot?Euler equation), or, equivalently, in terms of a ?rotational? pseudo-work?energy equation. Thermodynamical aspects in rotational systems, when e.g. frictional forces are present or when there is a variation of the rotational kinetic energy due to internal sources of energy, are discussed.

Thermodynamical asymmetries in whirling, jumping and walking

We analyze, from the thermodynamical point of view, mechanical systems in which there is the production of mechanical energy due to an internal source of energy, and compare that analysis with a similar one for the ?symmetric? motion that occurs with energy dissipation. The analysis of the energetic asymmetries is instructive to put in evidence the role of thermodynamics even in the discussion of mechanical aspects. We illustrate the discussion with the well-known example of a person on a rotating platform outstretching and contracting his or her arms, and also with other common situations such as jumping and walking.

A Demonstration Apparatus for the Cartesian Diver

The Cartesian diver is a nice toy and an intriguing physics instrument.1–6 Recently we reported an experimental study on the statics and dynamics of the Cartesian diver,7 using a specially designed apparatus that is much larger than the usual models. The Cartesian diver is an interesting example of the so-called “fold catastrophe,” the pressure being the control parameter,7 and this behavior is well observed in our apparatus.