Force-versus-time curves during collisions between two identical steel balls

We describe a numerical procedure for plotting the force-versus-time curves in elastic collisions between identical conducting balls. A system of parametric equations relating the force and the time to a dimensionless parameter is derived from the assumption of a force compatible with Hertz's theory of collision. A simple experimental arrangement consisting of a mechanical system of colliding balls and an electrical circuit containing a crystal oscillator and an electronic counter is used to measure the collision time as a function of the energy of impact. From the data we can determine the relevant parameters. The calculated results agree very well with the expected values and are consistent with the assumption that the collisions are elastic. (C) 2006 American Association of Physics Teachers.

Optimal space manoeuvres in a non-Keplerian force field

In this work the problem of a spacecraft bi-impulsive transfer between two given non coplanar elliptical orbits, with minimum fuel consumption, is solved considering a non-Keplerian force field (the perturbing forces include Earth gravity harmonics and atmospheric drag). The problem is transformed in the Two Point Boundary Value Problem. It is developed and implemented a new algorithm, that uses the analytical expressions developed here. A dynamics that considered a Keplerian force field was used to produce an initial guess to solve the Two Point Boundary Value Problem. Several simulations were performed to observe the spacecraft orbital behaviour by different kind of perturbations and constraints, on a fuel consumption optimization point of view. (C) 2002 COSPAR. Published by Elsevier B.V. Ltd. All rights reserved.

Oxidative dissolution of chalcopyrite by Acidithiobacillus ferrooxidans analyzed by electrochemical impedance spectroscopy and atomic force microscopy

The microbiological leaching of chalcopyrite (CuFeS2) is of great interest because of its potential application to many CuFeS2-rich ore materials. However, the efficiency of the microbiological process is very limited because this mineral is one of the most refractory to bacterial attack. Knowledge of bacterial role during chalcopyrite oxidation is very important in order to improve the efficiency of bioleaching operation. The oxidative dissolution of a massive chalcopyrite electrode by Acidithiobacillus ferrooxidans was evaluated by electrochemical impedance spectroscopy (EIS) and atomic force microscopy (AFM). A massive chalcopyrite electrode was utilized in a Tait-type electrochemical cell in acid medium for different immersion times in the presence or absence of bacterium. The differences observed in the impedance diagrams were correlated with the adhesion process of bacteria on the mineral surface. (C) 2004 Elsevier B.V. All rights reserved.

Grazing incidence X-ray diffraction and atomic force microscopy analysis of BaBi2Ta2O9 thin films

Thin films of BaBi2Ta2O9 (BBT) composition were prepared through the metal organic decomposition method. The crystallinity, phase formation, crystallite size and morphology of the thin films were measured as a function of the type of substrate, stoichiometry of solution and process variables such as thickness and temperature. The thin films were investigated by grazing incidence X-ray diffractometry and atomic force microscopy (AFM) techniques. For the sample without excess of bismuth, diffraction peaks other than that of the BBT phase were observed. A well crystallized BBT single phase was observed for films prepared from a solution with 10% excess of bismuth, deposited on Si/Pt substrate, with a thickness up to 150 nm and sintered at temperatures of 700 degreesC. The thin BBT phase films heat-treated at 600 degreesC presented a diffraction pattern characteristic of samples with lower degree of crystallinity whereas for the thin films heat-treated at 800 degreesC, we observed the presence of other phases than the BBT. For the thin film deposited on the Sin+ substrate, we observe that the peaks corresponding to the BBT phase are broader than that observed on the samples deposited on the Pt and Si/Pt substrates. No variation of average crystallite size was observed as the excess of Bi increased from 10 to 20%. AFM images for the samples showed that the increasing the amount of bismuth promotes grain growth. The average surface roughness measured was in the range of 16-22 nm showing that the bismuth amount had no or little effect on the roughness of films. (C) 2002 Elsevier B.V. B.V. All rights reserved.

Influence of the yielding criterion on total forming force in metallic junctions using elastomers

The production of metallic junctions employing elastomers is an unconventional technique that has been in development in the last 20 years. The forming process gets successfull just if a simultaneous compression between the elastomers and the tube takes place. Exact solutions for problems involving forming with elastomers are quite difficult to determine. However, the upper-bound theory can be used in order to predict the necessary load for junctions forming. Thus, it is necessary to develop a model capable to provide an estimate of the total forming force, which is useful to set-up tools and equipments required for the process. In this work, Von Mises, Hill's 1948 and Hill's 1979 associated yielding theories, and the Hosford's theory (1979) as well, were used in order to study the anisotropic behaviour on total forming force of junctions using elastomers, insuring the functionality of the proposed model. (c) 2006 Elsevier B.V. All rights reserved.

Retention force and fatigue strength of overdenture attachment systems

This study evaluated retention force and fatigue resistance of two overdenture attachment systems. Twenty samples (O-ring and Bar-Clip) from two manufacturers (Conexao Sistemas de Protese and Lifecore Biomedical) were prepared and divided into four groups: (i) Conexao/O-ring; (ii) Conexao/Bar-Clip; (iii) Lifecore/O-ring and (iv) Lifecore/Bar-Clip, with five samples in each group. They were submitted to mechanical fatigue test using a servohydraulic machine performing 5500 cycles of insertion and removal (f=0.8 Hz), immersed in artificial saliva. Retention force values were obtained three times (0, 3000 and after 5500 cycles) simulating the clinical service, using a tensile strength at 1 mm min(-1) and load cell of 1 kN. Data were analysed with analysis of variance and Tukey's test at 5% level. Results showed that Conexao/Bar-Clip specimens had significantly higher retention values than Lifecore/Bar-Clip (44.61 and 18.44 N, respectively), Conexao/O-ring specimens had significantly lower values than Lifecore/O-ring (13.91 and 19.75 N, respectively). Conexao/Bar-Clip values were always significantly higher than those of Conexao/O-ring group (44.61 and 13.91 N, respectively). Lifecore (O-ring and Bar-Clip) presented similar values (19.75 and 18.44 N, respectively). The systems evaluated showed satisfactory retention force values, before and after fatigue testing. Conexao/Bar-Clip specimens presented the highest values. A 5-year simulation of insertion and removal did not decrease retention values or fracture components.

Qualitative evaluation of active potential barriers in SnO2-based polycrystalline devices by electrostatic force microscopy

This paper discuss the qualitative use of electrostatic force microscopy to study the grain boundary active potential barrier present in dense SnO2-based polycrystalline semiconductors. The effect of heat treatment under rich- and poor-oxygen atmospheres was evaluated while especially considering the number of active barriers at grain boundary regions. The results show that the number of active barriers decrease after heat treatment in an oxygen-poor atmosphere and increase after heat treatment in oxygen-rich atmospheres. The observed effect was explained by considering the presence of oxidized transition metal elements segregated at grain boundary regions which leads to the p-type character of this region, in agreement with the atomic barrier formation mechanism in metal oxide varistor systems.

A new method for extending the range of conductive polymer sensors for contact force

This paper describes a technique for extending the force range of thin conductive polymer force sensors used for measuring contact force. These sensors are conventionally used for measuring force by changing electrical resistance when they are compressed. The new method involves measuring change in electrical resistance when the flexible sensor, which is sensitive to both compression and bending, is sandwiched between two layers of spring steel, and the structure is supported on a thin metal ring. When external force is applied, the stiffened sensor inside the spring steel is deformed within the annular center of the ring, causing the sensor to bend in proportion to the applied force. This method effectively increases the usable force range, while adding little in the way of thickness and weight. Average error for loads between 10 N and 100 N was 2.2 N (SD = 1.7) for a conventional conductive polymer sensor, and 0.9 N (SD = 0.4) using the new approach. Although this method permits measurement of greater loads with an error less than 1 N, it is limited since the modified sensor is insensitive to loads less than 5 N. These modified sensors are nevertheless useful for directly measuring normal force applied against handles and tools and other situations involving forceful manual work activities, such as grasp, push, pull, or press that could not otherwise be measured in actual work situations.

Ultrastructural and histochemical examination of alveolar bone at the pressure areas of rat molars submitted to continuous orthodontic force

It is usually believed that repair in alveolar bone during orthodontic movement occurs after decreasing of force. However, we have recently observed signs of repair in previously resorbed cementum from human teeth exposed to continuous forces. In order to test the hypothesis that bone resorption and deposition occur concomitantly at the pressure areas, a continuous 15 cN force was applied in a buccal direction to upper first molars from eight 2.5-month-old male Wistar rats for 3 d (n=4) and 7 d (n=4). As a control, two additional rats did not have their molars moved. Maxillae were fixed in 2% glutaraldehyde + 2.5% formaldehyde, under microwave irradiation, decalcified in ethylenediaminetetraacetic acid, and processed for transmission electron microscopy. Specimens from one rat from each group were processed for tartrate-resistant acid phosphatase (TRAP) histochemistry. At both the times studied, the alveolar bone surface at the pressure areas showed numerous TRAP-positive osteoclasts, which were apposed to resorption lacunae. In addition, osteoblasts with numerous synthesis organelles were present in the neighboring areas overlying an organic matrix. Thus, this study provides evidence that the application of continuous forces produces concomitant bone resorption and formation at the pressure areas in rat molars.

An analytical and numerical study of plane change maneuvers using aerodynamic force

The goal of the present work is to analyze space missions that use the terrestrial atmosphere to accomplish orbital maneuvers that involve a plane change. A set of analytical solutions is presented for the variation of the orbital elements due to a single passage through the atmosphere, assuming that the interval the spacecraft travels through the atmosphere is not too large. The study considers both the lift influence on the spacecraft orbit as well as drag. The final equations are tested with numerical integration and can be considered in accordance with the numerical results whenever the perigee height is larger than a critical value. Next, a numerical study of the ratio between the velocity increment required to correct the semimajor axis decay due to the atmospheric passage and the velocity variation required to obtain the change in the inclination is also presented. This analysis can be used to decide if a maneuver passing through the atmosphere can decrease the fuel consumption of the mission and, in the cases where this technique can be used, if a multiple passage is more efficient than a single passage.

Comment on Casimir force in compact non-commutative extra dimensions and radius stabilization

We call attention to a series of mistakes in a paper by S. Nam recently published in this journal (J. High Energy Phys. 10 (2000) 044).

Potential force of infection of human rabies transmitted by vampire bats in the Amazonian region of Brazil

Human rabies tansmitted by bats has acquired greater epidemiologic relevance in various Latin American countries, just when cases transmitted by dogs have decreased. Concern has been heightened by reports of increased rates of bats biting humans in villlages in the Amazonian region of Brazil. The aim of the present work was to estimate the potential force of infection (per capita rate at which susceptible individuals acquire infection) of human rabies transmitted by the common vampire bat if the rabies virus were to be introduced to a colony of bats close to a village with a high rate of human bites. The potential force of infection could be then used to anticipate the size of a rabies outbreak in control programs. We present an estimator of potential incidence, adapted from models for malaria. To obtain some of the parameters for the equation, a cross-sectional survey was conducted in Mina Nova, a village of gold prospectors in the Amazonian region of Brazil with high rates of bates biting humans. Bats were captured near dwellings and sent to the Rabies Diagnostic Laboratory at the Center for Control of Zoonoses (São Paulo, Brazil) to be examined. To estimate the force of infection, a hypothetical rabies outbreak among bats was simulated using the actual data obtained in the study area. of 129 people interviewed, 23.33% had been attacked by a vampire bat during the year prior to the study, with an average of 2.8 bites per attacked person. Males (29.41%) were attacked more often than females (11.36%); also, adults (29.35%) were attacked more often than children (8.33%). None of the 12 bats captured in Mina Nova tested positive for rabies, but the force of infection for a hypothetical outbreak was estimated to be 0.0096 per person per year. This risk represents 0.96 cases per 100 area residents, giving an incidence of 1.54 cases of bat-transimtted buman rabies per year in the village of Mina Nova (160 inhabitants). The estimated risk is comparable with what has been observed in similar Brazilian villages.