Francisco Vera

A piece of paper falling faster than free fall

We report a simple experiment that clearly demonstrates a common error in the explanation of the classic experiment where a small piece of paper is put over a book and the system is let fall. This classic demonstration is used in introductory physics courses to show that after eliminating the friction force with the air, the piece of paper falls with acceleration g. To test if the paper falls behind the book in a nearly free fall motion or if it is dragged by the book, we designed a version of this experiment that includes a ball and a piece of paper over a book that is forced to fall using elastic cords. We recorded a video of our experiment using a high-speed video camera at 300 frames per second that shows that the book and the paper fall faster than the ball, which falls well behind the book with an acceleration approximately equal to g. Our experiment shows that the piece of paper is dragged behind the book and therefore the paper and book demonstration should not be used to show that all objects fall with acceleration g independently of their mass.

Another Way of Tracking Moving Objects Using Short Video Clips

Physics teachers have long employed video clips to study moving objects in their classrooms and instructional labs. A number of approaches exist, both free and commercial, for tracking the coordinates of a point using video. The main characteristics of the method described in this paper are: it is simple to use; coordinates can be tracked using any standard Internet navigator; it can be used with examples available on our webserver or with a modified example in the users own computer; the web pages with new sequences of images are easy to upload to a web server; it is possible to track and analyze any complex one- or two-dimensional motion of objects; and the code is open source. The setup is very simple, intuitive, and can easily be scaled up.

Negative Pressures and the First Water Siphon Taller than 10.33 Meters

A siphon is a device that is used to drain a container, with water rising inside a hose in the form of an inverted U and then going down towards a discharge point placed below the initial water level. The siphon is the first of a number of inventions of the ancients documented about 2.000 years ago by Hero of Alexandria in his treatise Pneumatics, and although the explanation given by Hero was essentially correct, there is nowadays a controversy about the underlying mechanism that explains the working of this device. Discussions concerning the physics of a siphon usually refer to concepts like absolute negative pressures, the strength of liquid’s cohesion and the possibility of a siphon working in vacuum or in the presence of bubbles. Torricelli understood the working principle of the barometer and the impossibility of pumping water out of wells deeper than 10.33 m. Following Torricelli’s ideas it would also not be possible to build a siphon that drives pure water to ascend higher than 10.33 m. In this work, we report the first siphon that drives water (with surfactant) to ascend higher than the Torricellian limit. Motivated by the rising of sap in trees, we built a 15.4 m siphon that shows that absolute negative pressures are not prohibited, that cohesion plays an important role in transmitting forces through a fluid, and that surfactants can help to the transport of water in a metastable regime of negative pressures.

La Galería de Galileo: Videos de experimentos para la enseñanza de la Física

The Fondef TicEdu project “La Galeria de Galileo: Experimentos Interactivos de Fisica para la Ensenanza Media” aims to support the teaching-learning process in physics courses in public high-schools of Chile. Aiming to this purpose we built a gallery of experimental situations videos, along with inquiry-based guides, which will be used by teachers in their classes. In this way, the possibility of teaching Physics through the analysis of simple experiments makes it possible to alleviate in part the absence of laboratories in Chilean high schools, thus providing teachers with a tool that allows them to contextualize their classes and that allows the students to learn in an inquiry-based environment, making predictions, formulating hypothesis and testing them against the results of experiments.

Using Capacitors to Measure Charge in Electrostatic Experiments

In this work we describe a simple setup for measuring electrostatic charge and polarity in electrostatic experiments. This low-cost detector only requires a graphite pencil, a capacitor, two switches, and a voltmeter, and holds the promise of transforming qualitative activities that are commonly used to teach electrostatics into quantitative activities. In order to measure the charge and polarity of charged objects, a pencil is used as an antenna to collect charge from the object under study. The collected charge is transferred to a capacitor, and the resulting voltage at the capacitor is measured using a voltmeter. In contrast to other devices, our detector can be used to measure the amount of charge stored in objects at several thousand volts, as for example Van de Graaff domes.In this work we describe a simple setup for measuring electrostatic charge and polarity in electrostatic experiments. This low-cost detector only requires a graphite pencil, a capacitor, two switches, and a voltmeter, and holds the promise of transforming qualitative activities that are commonly used to teach electrostatics into quantitative activities. In order to measure the charge and polarity of charged objects, a pencil is used as an antenna to collect charge from the object under study. The collected charge is transferred to a capacitor, and the resulting voltage at the capacitor is measured using a voltmeter. In contrast to other devices, our detector can be used to measure the amount of charge stored in objects at several thousand volts, as for example Van de Graaff domes.

A Simple Alternative to the Phone Book Friction Demonstration

In this paper we describe a simple alternative to the telephone book friction experiment, a classic demonstration where a small force (generated for example by the bending of the outer pages) is amplified by the large number of surfaces in contact, resulting in a huge maximum static friction force that has to be counterbalanced in order to separate the books. The aim of our experiment is to illustrate the friction force in a dramatic way and to simplify the tedious job of interleaving a large number of book pages, by using the minimum number of sheets necessary to sustain the weight of a person using the normal force produced by their fingers.

Raltegravir plus abacavir/lamivudine in virologically suppressed HIV-1-infected patients: 48-week results of the KIRAL study

Background Long-term combination antiretroviral therapy often results in toxicity/tolerability problems, which are one of the main reasons for switching treatment. Despite the favorable profile of raltegravir (RAL), data on its combination with abacavir/lamivudine (ABC/3TC) are scarce. Based on clinical data, we evaluated this regimen as a switching strategy. Design Multicenter, non-controlled, retrospective study including all virologically suppressed HIV-1-infected patients who had switched to RAL+ABC/3TC. Methods We evaluated effectiveness (defined as maintenance of HIV-1-RNA <50 copies/mL at 48 weeks) safety, tolerability, laboratory data, and CD4+ count at week 48 of this switching strategy. Results The study population comprised 467 patients. Median age was 49 years (IQR: 45–53). Males accounted for 75.4%. Median CD4+ count at baseline was 580 cells/μL (IQR, 409). The main reasons for switching were toxicity/tolerability problems (197; 42.2%) and physician’s criteria (133; 28.5%). At week 48, HIV-1 RNA remained at <50 copies/mL in 371/380 (97.6%; 95%CI: 96.4–99.0) when non-virological failure was censured. Virological failure was recorded in 1.9% patients and treatment failure in 20.5% of patients (96/467 [95%CI, 16.9–24.2]). The main reasons for treatment failure included switch to fixed-dose combination regimens (31; 6.6%), toxicity/poor tolerability (27; 5.8%), and physician’s decision (17; 3.6%). A total of 73 adverse events were detected in 64 patients (13.7%). These resolved in 43 patients (67.2%). Of the 33 cases related or likely related to treatment, 30 were Grade-1 (90.9%). CD4+ count and renal, hepatic, and lipid profiles remained clinically stable over the 48 weeks. Conclusions Our findings suggest that RAL+ABC/3TC could be an effective, safe/tolerable, and low-toxicity option for virologically suppressed HIV-1-infected patients.

A simple experiment to measure the inverse square law of light in daylight conditions

The measurement of the irradiance of a point source of light as a function of the distance to the source is a nice and simple experiment for introductory physics courses. The resulting 1/r2 power law appears in other contexts and can be easily understood by the conservation of the total energy traversing concentric spherical surfaces of different radii. The typical verification of this law in the laboratory involves the use of a darkened room, a big restriction for this important experiment to be implemented in a modern inquiry based curriculum. To avoid this restriction, we designed a simple apparatus to measure the falloff of the irradiance with distance that can be used in daylight conditions.

A Galeria do Galileo: Vídeos de experimentos para ensino da física

The Fondef TicEdu project “La Galeria de Galileo: Experimentos Interactivos de Fisica para la Ensenanza Media” aims to support the teaching-learning process in physics courses in public high-schools of Chile. Aiming to this purpose we built a gallery of experimental situations videos, along with inquiry-based guides, which will be used by teachers in their classes. In this way, the possibility of teaching Physics through the analysis of simple experiments makes it possible to alleviate in part the absence of laboratories in Chilean high schools, thus providing teachers with a tool that allows them to contextualize their classes and that allows the students to learn in an inquiry-based environment, making predictions, formulating hypothesis and testing them against the results of experiments.

Galileo's Gallery: recordings of experiments for teaching Physics

The Fondef TicEdu project “La Galeria de Galileo: Experimentos Interactivos de Fisica para la Ensenanza Media” aims to support the teaching-learning process in physics courses in public high-schools of Chile. Aiming to this purpose we built a gallery of experimental situations videos, along with inquiry-based guides, which will be used by teachers in their classes. In this way, the possibility of teaching Physics through the analysis of simple experiments makes it possible to alleviate in part the absence of laboratories in Chilean high schools, thus providing teachers with a tool that allows them to contextualize their classes and that allows the students to learn in an inquiry-based environment, making predictions, formulating hypothesis and testing them against the results of experiments.