Determinació experimental de l’acceleració de la gravetat mitjançant la targetade so d’un ordinador

Résumé

Physics is an empirical science whose purpose is to understand the laws that govern nature through the analysis and description of the phenomena that occur in it. Scientific research is based on thescientific method, through the observation and the approach of a certain problem, the proposal of hypothesis and the experimentation, where the hypotheses are verified or not. The scientific training of students should include experimental activities, and laboratory practices should be a fundamental pillar of scientific education. However, there is a general lack of interest in the study and understanding of the experimental sciences and, in particular, of Physics. It is therefore essential to use new methods, different and/or complementary to the traditional ones, to motivate studentsin their interest in science. These resources include the performance of recreational experiments of physics in conjunction with the use of new technologies. During the last years, the technological development has been spectacular, becoming a new culturalphenomenon with a significant impact on our society. Today young people are often very motivated by the use and learning of new technologies, so their use as pedagogical tools in teaching physics has a great potential and represents an interesting challenge.The goal of this project is to do experiments in order to determine the acceleration of gravity by means of the program Audacity. This is a free software that young people use often to record, reproduceand edit sounds. We will employ this program to register, process and analyse acoustic signals. The acquisition device is the microphone (or other device connected to the computer sound card) andallows recording very precisely time intervals between two acoustic signals. In the first experiment we measure the fall time of an object in order to calculate the value of the acceleration due to gravity. When an object falls from a height of the order of 1 meter, it takesapproximately half a second to reach the ground. This falling time is too short to measure with a stopwatch, since the reaction time of a person is approximately 0.2 s. An alternative way of measuringsmall time intervals is to record audio signals and then analyse the acoustic signal. To carry out this experiment we have built a rigid pendulum that strikes laterally a glass marble resting at the edge ofa table, producing a sonic pulse; then the marble falls emitting another sonic pulse when it reaches the ground. The Audacity program registers the acoustic signal as a function of time, which allows determining the time of fall of the marble. As the movement of the marble is uniformly accelerated, knowing the initial height of the marble, we can obtain the value of the acceleration due to gravity.The second experiment consists of dropping a magnet through a glass or plastic tube having several copper coils (small solenoids), which are situated at different points and connected to each otherand to the audio input of the computer. Due to Faraday’s electromagnetic induction law, an induced electromotive force is produced in the solenoids when the magnet goes through. The induced electromotive force is registered by the Audacity software, determining in this manner the time that the magnet took to pass between consecutive solenoids. Taking into account that the magnet is subjected to a uniformly accelerated movement, we can obtain the gravity acceleration.