The 'spinning disk touches stationary disk' problem revisited: an experimental approach

A popular Newtonian mechanics problem, featured in textbooks, physics olympiads and forums alike, concerns two disks with different radii and moment of inertia that rotate differently and that touch each other. Most students struggle to calculate the final angular velocity of the disks, erroneously attempting to use different conservation laws. In this paper we propose a simple experiment that should help physics teachers explain this challenging exercise in an engaging way for the students. By using a smartphone/tablet and video analysis tools, the angular velocity of both disks can easily be tracked as a function of time, clearly showing the three stages of the interaction (before touching, only one disk rotating; touching with slippage; and touching without slippage). Processing and plotting of the data in a spreadsheet immediately shows which quantities are conserved and which are not. Several extensions to the core experiment are also suggested

The effect of a tangential frictional force on rotating disks: an experimental approach

This paper describes an experiment with two touching rotating disks, whose movement is followed by video analysis. Within the disks’ movements, there are intervals with sliding and intervals without sliding, that is, intervals with frictional forces between the touching surfaces and intervals without it. This system configuration allows for measurement of the changeable magnitudes and directions of frictional forces (much more difficult to set up with translational motion). This activity may be used to combat students misconceptions of “frictional force always opposes the motion” and “between the same two bodies, one gets the same frictional force,” commonly found in the classroom, or just to reinforce the rotational dynamics relationships

Relação entre fatores abióticos e emissão de CO2 em sistemas agroflorestais com dendê em Tomé-Açu - PA.

Apesar da importância do bioma Amazônico tanto por abrigar grande biodiversidade quanto por influenciar no regime de chuvas, esta região tem sido submetida a queimadas e a mudanças no uso da terra. Nesse contexto, os sistemas alternativos de cultivo, que buscam a produção sustentável ganham espaço, como os Sistemas Agroflorestais. Este trabalho teve como objetivo analisar a relação entre a emissão de CO2 pelo solo, a temperatura e a umidade do solo em dois sistemas alternativos de cultivo de dendê, adubadeira e biodiverso, em Tomé-Açu ? PA, na área onde está sendo desenvolvido o projeto Projeto Dendê em Sistemas Agroflorestais na Agricultura Familiar. O monitoramento de CO2, temperatura e umidade do solo foi realizado no período de dezembro de 2010 a maio de 2011, em 36 pontos em cada tratamento. A análise de Correlação de Pearson foi feita para verificar se existe correlação entre o dióxido de carbono e esses fatores abióticos, também uma análise de regressão foi realizada para gerar um modelo que mostre esta relação. Foi verificada correlação linear positiva entre CO2 e temperatura do solo nos dois tratamentos analisados, e correlação linear negativa entre o gás carbônico e a umidade do solo no sistema adubadeiras. No sistema biodiverso não foi verificada relação entre esses dois componentes. Pode-se concluir que os fatores abióticos analisados interferem de forma fraca no efluxo de CO2 do solo, no período estudadoEditores técnicos: Roberto Porro, Milton Kanashiro, Maria do Socorro Gonçalves Ferreira, Leila Sobral Sampaio e Gladys Ferreira de Sousa

Development and application of I_SUM : an index of sustainable urban mobility

This work describes the construction process of an Index of Sustainable Urban Mobility (I_SUM). Important aspects regarding its application as a mobility planning and management supporting tool are also discussed. The index is based on a hierarchy of criteria built essentially with data obtained from planners and technicians of eleven cities, what resulted in eighty-seven Indicators forming thirty-seven Themes, under nine Domains. Weights for the Themes and Domains were obtained through a panel of experts. A viability analysis of the index application was carried out in a city with around 220,000 inhabitants located in the state of São Paulo, Brazil. The analysis of the results has shown that a significant number of indicators could be calculated in the short term and with good quality data in the studied city. The main conclusion drawn from the application was the confirmation that I_SUM is flexible enough to be applied not only in a developing country medium-sized city, but also in cities with diverse characteristics. It is important to highlight the fact that the tool also gives the opportunity to assess the relative contribution of any specific aspects of mobility to the Social, Economic, and Environmental dimensions of sustainability. That I_SUM feature can be used to help in the formulation of either integrated or stand-alone policies per sector aiming at the development of the sustainable urban mobility. Moreover, it can help in the public participation process due to the possibility of building up scenarios based on hypothetical changes of the indicator values