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Students learn physics better if they are involved in active learning modes, consisting of different sequences of hands-on and minds activities. The usual way of teaching weightlessness is not adequate for students’ active learning. The concept is typically introduced through “thought experiment” with a free-falling lift in which a person standing on a balance reads “zero weight”. Although some physics textbooks ask students to explain why a water jet stops to flow out of a bottle in free fall, we couldn’t find any experimental study concerned with students’ explanations of this particular demonstration of weightlessness. Therefore, we designed and carry out a corresponding research with students of primary and high schools from Bosnia and Herzegovina and Serbia. The students were asked to explain in a paper-and-pencil format why water flows out of a bottle with a hole when the bottle is at rest? Why does water stop flowing out when the bottle is in free-fall? Propose an experiment that would show that your explanation is right. What should happen in that experiment and why? The students’ answers and drawings revealed many different alternative explanations. They show that active learning of weightlessness might be possible in physics classrooms.

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Different "thought experiments" dominate teaching approaches to weightlessness, reducing students' opportunities for active physics learning, which should include observations, descriptions, explanations and predictions of real phenomena. Besides the controversy related to conceptual definitions of weight and weightlessness, we report another controversy regarding the position of the person that weighs herself or himself in a freely-falling elevator, a "thought experiment" commonly used for introducing the concept of weightlessness. Two XIX-century "thought experiments", one from America and one from Russia, show that they have a long tradition in physics teaching. We explored experimentally a "thought experiment" that deals with the behavior of a mercury drop in a freely-falling elevator. Our experimental results show that the mercury drop neither took the expected spherical shape nor performed oscillatory motions predicted by theory. Teachers should encourage students to enrich active learning of weightlessness by thinking how to test experimentally the answers to some conceptual questions, a subclass of "thought experiments".

[Recibido en marzo de 2015, aceptado en julio de 2015] Es bien conocido que un chorro de agua, que fluye de un recipiente perforado que está en reposo, deja de fluir si el recipiente se deja en caída libre. En este artículo, se describe en detalle este fenómeno. Para lograr ese fin, hemos montado a una plataforma un recipiente transparente, del que fluye un chorro de agua, y una cámara que puede realizar vídeograbaciones. Cuando se deja caer libremente la plataforma, se observa cómo desaparece el chorro inicial. Se describen los fenómenos observados durante el proceso de caída y se explican en forma cualitativa. Se sugieren posibles usos didácticos de las fotos capturadas para promover el aprendizaje activo de la física. How does a water jet stop to flow out from a free-falling can?It is well known that a water jet, flowing out from a vessel with a hole that is in rest, stops to flow out if the vessel is in free fall. In this article, we describe a few interesting details of that cease of jet's flow out. To carry out that aim, we have set up on a platform a transparent vessel, from which a water jet flows, and a camera that can do video-recordings. When the platform is leaved to fall freely, it was possible to film the details of how the initial jet disappears. Various observed phenomena are described and commented. These phenomena are explained in a qualitative manner, using physics concepts. Possible didactic uses of the captured photos promoting active physics learning are suggested. IntroducciónEl comportamiento poco común de los astronautas y diferentes objetos en las naves espaciales, gracias a numerosos vídeos de la NASA y otras agencias espaciales disponibles en YouTube, se hizo parte de la cultura visual contemporánea. Como ejemplos ilustrativos se pueden mencionar, el comportamiento de una esfera de agua causado por diferentes acciones externas y el de la llama de una vela, entre otros (NASA 2012).Esos fenómenos se toman como evidencias de que tanto agua y astronautas como llamas de vela están en el «estado de ingravidez». Sin embargo, la interpretación de tal concepto es controvertida, pues depende de cómo se define el concepto de peso. Las dos maneras principales son la «gravitacional» y la «operacional» (Galili 1995).Si el peso de un cuerpo se define como la fuerza gravitacional que ejerce la Tierra (u otro planeta) sobre el cuerpo, entonces, se trata de «ingravidez aparente». Aunque el cuerpo y la nave están bajo la influencia de la gravitación terrestre, el cuerpo en la nave espacial, en caída libre, se comporta como si su «peso aparente» (la fuerza de contacto que ejerce, por ejemplo, sobre una balanza) fuera cero.Si, por otro lado, el peso se define, de manera operacional, como la fuerza con que el objeto presiona la balanza, entonces los cuerpos en la nave espacial, estando todos en caída libre, no pueden ejercer tal fuerza de presión y se encuentran en el estado de «ingravidez real». Los