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Feeling Laplace

2011; American Physical Society; Volume: 35; Issue: 1 Linguagem: Inglês

10.1152/advan.00061.2010

1522-1229

Hamilton Haddad, Ivana Trindade Sá Brito,

Science Education and Pedagogy

IlluminationsFeeling LaplaceHamilton Haddad, and Ivana BritoHamilton HaddadDepartment of Physiology, Biosciences Institute, University of São Paulo, São Paulo; and , and Ivana BritoDepartment of Physiology, Metropolitan University of Santos, Santos, BrazilPublished Online:01 Mar 2011https://doi.org/10.1152/advan.00061.2010MoreSectionsPDF (342 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations The law of Laplace links the tension in the walls of a container with its radius and the pressure of its contents. This relationship (tension is proportional to the product of radius × pressure) has a broad implication in physiology and medicine, such as pulmonary and circulatory mechanics, bladder function, intraocular pressure, pregnancy, heart failure, hypertension, and several other conditions (1, 2). Students' difficulty in understanding Laplace's law frequently lies in the arid way it is presented: hastily in respiratory physiology courses, in which the only emphasis is given on the role of surfactant in alveolar collapse, despite the fact that the law probably has little to do with alveoli collapse (4). Little attempt is dedicated to its underlying principles or to its role in linking anatomic design to the functional capability of diverse body structures. Here, we describe a quite simple way for physiology students to figure out the physical forces involved in Laplace's law. The novelty proposed here is to make students somatically “feel the law,” by means of touch and proprioception. An ever-growing body of evidence has shown us the importance of this kind of active and meaningful learning (3).In this demonstration, the instructor places two students one in front another, keeping a distance of ∼1.5 yd between them. Both students have to firmly hold the extremities of a piece of fabric (a towel, for example, as shown in Fig. 1). In the middle of the fabric, the instructor should place an object weighing ∼10 lb. A student's purse or a backpack is a good choice. The instructor then asks students to compare how difficult is to hold the fabric in two distinct situations (as shown in Fig. 1) and report it to the rest of the class. The object weight simulates the pressure of a content (like a fluid) against the walls of a given body structure (a blood vessel, for instance). The difficulty of holding the fabric is associated with the tension stress in the curved wall of that structure, a tangential force to its surface. Students will easily feel that the tension in the fabric changes as a function of the distance between them. The closer they are, the lesser is the tension; consequently, the easier it is to hold the fabric (and vice versa). As the pressure (object weight) remains constant along the demonstration, tension increases and decreases as a function of the radius of the imaginary circle shaped by the fabric: tension is proportional to pressure × radius. After the demonstration, the instructor can discuss with the class about what happened. The perpendicular component of the tension in the curved wall counterbalances the pressure, and the size of this component varies as a function of radius.Fig. 1.The vertical component (Ty) of the tension (T1 and T2) cancels the weight [W; which simulates the pressure against the wall of container (the imaginary circle formed by the fabric)]. A: when the students are close together, given the small radius, a smaller tension is needed to produce the vertical counterbalancing component, as in the following relation: tension is proportional to pressure × radius. B: as the distance between the two students increases, the radius of the imaginary circle increases as well, increasing the angle between the tension made by each student separately. Given the vectorial nature of forces, a greater tension is needed to generate the same vertical component (T2 > T1), which is vividly felt by the students. The instructor should point that the object weight (pressure) remains the same throughout the demonstration.Download figureDownload PowerPointThe law of Laplace can provide valuable insights into the function of many of the body's systems. However, it often leads to counterintuitive predictions about the behavior of hollow structures that work under pressure. It has been shown that students have serious misconceptions about this kind of behavior (5). We performed this simple demonstration in our introductory physiology courses with great results and extremely positive feedback from the students.DISCLOSURESNo conflicts of interest, financial or otherwise, are declared by the author(s).ACKNOWLEDGMENTSThe authors thank the students in the photos and Prof. Marcus Vinicius C. Baldo for helpful discussions on the central topic of this article.REFERENCES1. Basford JR. The law of Laplace and its relevance to contemporary medicine and rehabilitation. Arch Phys Med Rehabil 83: 1165–1170, 2002.Crossref | PubMed | ISI | Google Scholar2. Mark HH. The role of eye size in its pressure and motility. Clin Ophthalmol 1: 105–109, 2007.Google Scholar3. Michael J. Where's the evidence that active learning works? Adv Physiol Educ 30: 159–167, 2006.Link | ISI | Google Scholar4. Prange HD. Laplace's law and the alveolus: a misconception of anatomy and a misapplication of physics. Adv Physiol Educ 27: 34–40, 2003.Link | ISI | Google Scholar5. Silverthorn DU. Using demonstrations to uncover student misconceptions: the Law of Laplace. Adv Physiol Educ 22: 281–282, 1999.Link | ISI | Google ScholarAUTHOR NOTESAddress for reprint requests and other correspondence: H. Haddad, Dept. of Physiology, Biosciences Institute, Univ. of São Paulo, Rua do Matão, Travessa 14, 101, São Paulo SP 05508-900, Brazil (e-mail: hamilton.[email protected]com). Download PDF Previous Back to Top Next FiguresReferencesRelatedInformation Cited ByStatus of research on physiology education in BrazilRui Seabra Machado and Pâmela Billig Mello-Carpes7 September 2018 | Advances in Physiology Education, Vol. 42, No. 4Learning style-based teaching harvests a superior comprehension of respiratory physiologyAnbarasi M, Rajkumar G, Krishnakumar S, Rajendran P, Venkatesan R, Dinesh T, Mohan J, and Venkidusamy S1 September 2015 | Advances in Physiology Education, Vol. 39, No. 3Feeling wall tension in an interactive demonstration of Laplace's lawMilorad Letić1 June 2012 | Advances in Physiology Education, Vol. 36, No. 2 More from this issue > Volume 35Issue 1March 2011Pages 97-98 Copyright & PermissionsCopyright © 2011 the American Physiological Societyhttps://doi.org/10.1152/advan.00061.2010PubMed21386009History Received 7 June 2010 Accepted 22 December 2010 Published online 1 March 2011 Published in print 1 March 2011 Metrics