A dynamical model of remote-control model cars

Simple experiments for which differential equations cannot be solved analytically can be addressed using an effective model that satisfactorily reproduces the experimental data. In this work, the one-dimensional kinematics of a remote-control model (toy) car was studied experimentally and its dynamical equation modelled. In the experiment, maximum power was applied to the car, initially at rest, until it reached its terminal velocity. Digital video recording was used to obtain the relevant kinematic variables that enabled to plot trajectories in the phase space. A dynamical equation of motion was proposed in which the overall frictional force was modelled as an effective force proportional to the velocity raised to the power of a real number. Since such an equation could not be solved analytically, a dynamical model was developed and the system parameters were calculated by non-linear fitting. Finally, the resulting values were substituted in the motion equation and the numerical results thus obtained were compared with the experimental data, corroborating the accuracy of the model.Comment: 5 pages, 9 figure

An experiment to address conceptual difficulties in slipping and rolling problems

A bicycle wheel that was initially spinning freely was placed in contact with a rough surface and a digital film was made of its motion. Using Tracker software for video analysis, we obtained the velocity vectors for several points on the wheel, in the frame of reference of the laboratory as well as in a relative frame of reference having as its origin the wheel’s center of mass. The velocity of the wheel’s point of contact with the floor was also determined obtaining then a complete picture of the kinematic state of the wheel in both frames of reference. An empirical approach of this sort to problems in mechanics can contribute to overcoming the considerable difficulties they entail

Video-based analysis of the transition from slipping to rolling

The problem of a disc or cylinder initially rolling with slipping on a surface and subsequently transitioning to rolling without slipping is often cited in textbooks [1-2]. Students struggle to qualitatively understand the difference between kinetic and static frictional forces—i.e., whereas the module of the former is known, that of the latter can only be described in terms of an inequality while the relative velocity at the point(s) of contact is equal to zero. In addition, students have difficulty understanding that frictional forces can act in the direction of motion—i.e., they can accelerate object

Normal coordinates in a system of coupled oscillators and influence of the masses of the springs

Experimental analysis of the motion in a system of two coupled oscillators with arbitrary initial conditions was performed and the normal coordinates were obtained directly. The system consisted of two gliders moving on an air track, joined together by a spring and joined by two other springs to the fixed ends. From the positions of the center of mass and the relative distance, acquired by analysis of the digital video of the experiment, normal coordinates were obtained, and by a non linear fit the normal frequencies were also obtained. It is shown that although the masses of the springs are relatively small compared to that of the gliders, it is necessary to take them into consideration to improve the agreement with the experimental results. This experimental-theoretical proposal is targeted to an undergraduate laboratory

What are the attitudes and beliefs about Science of the Physics teachers and future Physics teachers in Uruguay?

We investigated the epistemological conceptions of the uruguayan Physics teachers and future Physics teachers through the application of the CLASS test (Colorado Learning Attitudes about Science Survey), one of the most accepted instruments in the research community in Physics education. The results obtained allow us to compare the attitudes and beliefs about science of both groups and quantitatively evaluate the agreement or not with the conceptions of experts in the field. First, we present a general screenshot of the responses and then we identify categories in which there are significant similarities or differences between the two groups studied and in turn with the reference of the experts. The categories that show significant positive or negative variations between the opinions of future teachers and teachers indicate the areas where training is favorable or unfavorable. On the other hand, the areas where the differences with the opinions of the experts are globally notorious suggest that they should be strengthened. To get a more global perspective, we also compare our results with those published in the literature. Finally, we raise several questions that we think may favor further inquiries.Comment: 25 pages, Spanish and English version

Differences in the attitudes and beliefs about science of students in the physics-mathematics and life sciences areas and their impact on teaching

For this study, we compared the attitudes and beliefs about science of physical science (physics and mathematics) and life science (bio- chemistry and biology) students at the beginning of their university degrees using the CLASS (Colorado Learning Attitudes about Science Survey) tool. It is worth noting that both groups of students received similar physics courses during their high-school education. Through a detailed analysis of the different categories of the test, we examined the differences in performance in each of the areas that make up the questionnaire. Among other aspects, we found that a considerable percentage of life science students (higher than that of physical science students) adopted a novice type of behavior in problem solving. Finally, we discussed the possible causes of the differences found and their implications for teaching

Reversal of the hip fracture secular trend is related to a decrease in the incidence in institution-dwelling elderly women

Summary: In this prospective 10-year study in elderly aged 60years and over, there was a 1.3% per year reduction in the standardized incidence of hip fracture in women but not in men. This decrease was mainly due to changes in the standardized incidence of hip fracture in institution-dwelling women. Introduction: A decrease in age-adjusted hip fracture incidence has been recently demonstrated in some countries. Since a large proportion of hip fractures occur in nursing homes, we analyzed whether this decreasing trend would be more detectable in institution-dwelling elderly compared with community-dwelling elderly. Methods: All hip fracture patients aged 60years and over were identified in a well-defined area. Incidence of hip fracture, age- and sex-adjusted to the 2000 Geneva population, was computed in community- and institution-dwelling elderly. Results: From 1991 to 2000, 1,624 (41%) hip fractures were recorded in institutionalized-dwelling elderly and 2,327 (59%) in community-dwelling elderly. The standardized fracture incidence decreased by 1.3% per year in women (p = 0.039), but remained unchanged in men (+0.5%; p = 0.686). Among institution-dwelling women, hip fracture incidence fell by 1.9% per year (p = 0.044), whereas it remained stable among community-dwelling women (+0.0%, p = 0.978). In men, no significant change in hip fracture incidence occurred among institution- or community-dwelling elderly. Conclusions: The decrease in the standardized hip fracture incidence in institution-dwelling women is responsible for the reversal in secular trend. Future research should include stratification according to the residential status to better identify the causes responsible for the trend in hip fracture incidenc