O problema da máquina de Atwood com massa variável é apresentado neste trabalho através de uma configuração robusta, que permite a aquisição de dados da dinâmica do movimento oscilatório durante um intervalo de tempo relativamente longo. O sistema é facilmente implementado através da utilização de uma polia e um conjunto de três corpos cujas massas obedecem a relação M>m1>m2 e M<m1+m2. O problema pode ser tratado de maneira lúdica para alunos do ensino médio ou com uma profundidade maior, através da utilização do formalismo Lagrangiano, para estudantes de nível superior. O movimento oscilatório do sistema não se assemelha com nenhum dos casos de amortecimento periódico ou aperiódico observado na literatura, pois a massa do sistema varia em intervalos de tempo não constantes. Através da utilização de fotogates (sensores) acoplados à polia e do software Tracker obteve-se um conjunto de variáveis dinâmicas para comparação com os resultados esperados oriundos da descrição analítica.

An Atwood machine with variable mass is presented in this work using a robust configuration that enables data acquisition from the dynamic oscillatory motion during a relatively long time interval. The system is easily implemented through the use of a pulley and a set of three bodies whose masses satisfy the relationship M>m1>m2, and M<m1 + m2. The problem can be treated in a playful way for high school students or with greater depth, using the Lagrangian formalism for undergraduate students. The oscillatory motion of the system is not similar to any of the cases of periodic or aperiodic damping reported in the literature, because the mass of the system changes with decreasing time intervals. Through the use of photogates (sensors) coupled to the pulley and Tracker Software it was obtained a set of dynamic variables to be compared with the expected results derived from the analytical description.

Este artigo trata do estudo de um sistema dinâmico de massa variável onde se implementa o experimento da corrente de elos cuja extremidade está unida através de um fio a um corpo suspenso na máquina de Atwood. O movimento harmônico amortecido do sistema foi observado. O princípio fundamental da dinâmica foi empregado na obtenção da taxa de variação temporal da aceleração linear do movimento ascendente da corrente. O emprego de uma filmadora digital no processo experimental possibilitou a caracterização do movimento através do registro da posição dos corpos envolvidos em função do tempo, de uma maneira simples e econômica. Um software de domínio público (VirtualDub) foi usado para a seleção dos quadros da filmagem. As variáveis dinâmicas obtidas experimentalmente foram confrontadas com os resultados esperados baseados na teoria, com boa concordância. Comparações foram feitas entre estes resultados com os obtidos através do emprego de fotogates (sensores) comerciais dispendiosos. A ineficácia dos fotogates foi verificada na coleta das variáveis dinâmicas no início do movimento.

This paper deals with the study of a variable mass dynamic where is developed the Atwood machine experiment with variable mass of a resting chain in which one end is attached to a fine string which passes over a pulley and carries a weight. The damped harmonic oscillation was observed. The Newton's second law was employed to obtaining of the upward motion of the chain. Using a digital camera and free software VirtualDub it was possible to record the evolution of the space-time and to reproduce it in a number of frames in a low cost operation. Good agreement was obtained between experimental results and the predictions of the movement dynamical variables. Comparisons between these results and those obtained using expensive photogates were done. These photogates shown bad results of the measurements done at the beginning of the movement.

Microdischarges at moderate to high pressure in argon were investigated. A hole opening diameter of 500 µm direct current (dc) microhollow cathode discharges (MHCD) were characterized by electrical measurements and optical emission spectroscopy (OES) for pressures ranging between 90 and 800 Torr and current from 5 to 20 mA. Current-voltage characteristic curves were obtained as a function of the pressure for this hole diameter. MHCD enables stable dc discharges for molybdenum electrodes material at constant Ar + 2%H2 flow of 0.03 l/min. Optical emission spectroscopy and analysis of the spectral line broadening of plasma line emissions were performed in order to measure gas discharge parameters. Electron number densities were obtained from Hβ Balmer line ( ~ 10(14) cm-3). For the above mentioned discharge conditions, gas temperature was estimated to be 550 - 850 K from OH rotational bands. Excitation temperature was measured based on two lines method (from atomic Mo lines) and from 4p - 4s and 5p - 4s Ar radiative transitions. Hydrogen atom temperature was measured for 800 Torr ( ~ 12000 K).

Através de uma filmadora digital e do software de domínio público VirtualDub consegue-se registrar espacial e temporalmente a evolução dinâmica de um sistema muito rápido, como por exemplo, a queda livre. Neste artigo são abordados dois experimentos clássicos de queda livre: a) de uma corrente de elos e b) de uma configuração catenária da corrente com uma das extremidades fixa (bungee jump). Estes experimentos são analisados empiricamente através de montagens bastante simples, mas reveladores de conceitos físicos importantes da mecânica. O programa VirtualDub possibilita selecionar os quadros da filmagem. A análise destes quadros fornece prontamente elementos das diversas variáveis dinâmicas tais como velocidade, força e aceleração, sendo um poderoso auxiliar no estudo do movimento.

Using a digital camera and public common software VirtualDub it was possible to record the space-time evolution of a fast dynamic system, such as a body in free fall. This paper treats two classical free fall experiments: a) falling chain and b) a free falling folded chain (bungee jump). Analyzed empirically in a simple setup, these experiments showed very important concepts of mechanics. The VirtualDub program enables the selection of frames from the video record. The analysis of these frames provides elements of several dynamical variables such as velocity, force and acceleration, and is a powerful tool in the study of the movement.

This work is devoted to the study of the electrical characteristics of microhollow cathode discharges (MHCD) at moderate to high pressure in argon and air for different geometries. High-pressure glow discharges can be operated in MHCD devices with µm electrode spacing. Experiments have been performed to determine the so-called Paschen curves, i.e. the dependence of the breakdown voltage on the product electrode gap and gas pressure. Current-voltage characteristic curves were obtained as a function of the pressure and hole diameter. MHCD enables stable direct current discharges that could be ignited for pressures ranging from 12 to 800 Torr, in a very wide range of current densities and electrodes materials. Evidence of electron field emission was observed for several ranges of gap spacing.

The electrical breakdown has been investigated for low-pressure argon and nitrogen discharges under the influence of an external longitudinal magnetic field. Plane-parallel aluminum electrodes (5 cm diameter) separated by a variable distance d (4.0 cm < d < 11.0 cm) were sustained with a dc voltage (0 < V < 1 kV). A Helmholtz coil was used to produce an uniform magnetic field(B) parallel to the discharge axis. Paschen curves were obtained and the secondary electron emission coefficient (gamma), the first Townsend ionization coefficient (alpha) and the ionization efficiency(eta), were plotted with respect to the variation of the reduced field (E/P). To observe the effect of the magnetic field these curves were plotted for fixed values of B=0 and B=350 Gauss. As consequence of the longitudinal magnetic field, the free paths of the electrons in the Townsend discharge are lengthened and their lateral diffusion is reduced, thus reducing electron losses to the walls. The data presented in this paper give a quantitative description of the B-field effect on the Townsend's coefficients and overall it is concluded that the DC electrical breakdown of the gases is facilitated if a longitudinal magnetic field is applied along the discharge axis.