Teaching physics by magic

PAPER iopscience.org/ped Phys. Educ. 54 (2019) 015025 (15pp) Teaching physics by magic Franco Bagnoli1,3 , Alessio Guarino2 and Giovanna Pacini1 1 Department of Physics and Astronomy and CSDC, University of Florence via G. Sansone 1, 50019 Sesto Fiorentino, Italy 2 Laboratoire Icare, Université de la Réunion, St Denis de la Réunion, Réunion, France 3 INFN, Sez. Firenze, Italy E-mail: [email protected] Abstract In this paper we describe the results of some experiments about using surprising physics demonstrations, presented as magical phenomena followed by scientific explanations, for introducing physics topics in several teaching contexts. All the demonstrations have been designed to be implemented with easy to get and cheap materials, so that students can reproduce them at home. This approach has been exploited in Italian high schools, Italian elderly people education and French primary schools, with good results. this approach is quite different from that of analyzing standard ‘magics tricks’ from the point of view of physics [6]. In the last years, we have developed a set of simple demonstrations that can be repeated at home using cheap equipments and scrap materials. We have tested this approach in several contexts, in schools of various levels and universities, in dissemination events, in courses for elderly people and in public exhibitions. We found promising results for what concerns the participation, the engagement of attendees and learning results. In this paper we present some of our demonstrations, that have been tested in a cycle of lessons for high school students in all Tuscany, in lessons for elderly people (University of Free Age, Municipality of Florence) and in a class for teacher preparation in Reunion, and the results of these pedagogical and didactic experiences. 1. Introduction Physics is a difficult subject! This is the most common statement by students and lay people about this topic. Indeed, it is true! The main problem with physics is that it does not rely on what should be learnt, but in what has to be forgotten or contextualized. Our body, and therefore our brain, has been selected to successfully deal with everyday experiences, and to comply with them we are born with a hard-wired general knowledge of ‘real-life physics’, which is essentially Aristotelian [1] or possibly medioeval [2]. This innate knowledge is also shared with other animals [3]. Unfortunately, a deeper inspection reveals that the world follows other rules, and therefore the main goal of a teacher is that of inducing pupils to switch (depending on the context) to a different reasoning path with respect to the innate one. One technique that can be used for this goal is that of ‘surprising’ the audience [4] by means of nearly-magic experiences, followed by a discussion, with an analysis of the physics principles involved and the illustration of possible extensions, fields of application, etc [5]. With respect to standard ‘science fair’ experiments, we mainly focus on the magic in everyday life. Notice that 1361-6552/19/015025+15$33.00 2. Demonstrations We perform a variable number of demonstrations, but with a common set of ten experiments (table 1). Let us review them, including some description of the narrative context used and references to the discussion of the physical content of the demonstration. Clearly, the demonstrations and the 1 © 2018 IOP Publishing Ltd F Bagnoli et al Table 1. The 10 most common demonstrations. Title Physical principles Sinking the Titanic Diver in a bottle Water in the net Balancing broom Bouncing balls Drinking bird Archimedes’ principle Archimedes’ principle, pressure, Boyle’s law, Stevino’s law Air pressure, surface tension Torsion, center of mass, friction Potential energy, kinetic energy, elastic collision, reference system Second law of thermodynamics (Kelvin statement), evaporation, relative humidity, condensation, pressure, heat, center of mass, thermal machine, entropy Tea leaves/winding rivers Centrifugal force, fluid balance, connecting pipes, transverse flow Obedient balloon Bernoulli’s law, equilibrium pressure, pressure of a fluid in motion, venturi effect Roberto Carlos Bernoulli’s law, magnus effect, viscous drag, reference system Vortex cannon/smoke Bernoulli’s law rings narrative parts evolved over time, and we are here presenting the latest version. We tried to stimulate comments and discussions, pointing to the unexpected connections among apparently very different disciplines. The common ‘catchphrase’ (that attendees are asked to shout together) after each demonstration is ‘why?’. and simulate the Titanic with a glass (preferably with a thick bottom). Mark the water (sea) level and ask the public to predict the fate of that level after the sinking of the ship. Will it raise? Will it remain the same? Will it lower? Typically, a large majority of attendees in the public vote for the first option, and remains quite surprised seeing that the ‘sea level’ in effect lowers when the ship sinks (figure 1). We can profit from this opportunity by having the public recite Archimedes’ principle, for which a body immersed in a fluid receives a thrust equal to the weight of the displaced fluid. Since vessels are made of a material heavier than water, they must displace more water than the volume of the material they are made with, once sunk they only displace the latter volume. Therefore, a big sinking ship makes a ‘hole’ in the ocean, and the surrounding water rushes in to fill it, dragging the unfortunate shipwrecked. Many other narrations about Archimedes’ law are possible. For instance, when Leonardo da Vinci arrived in Milan in 1482, the city was crossed by various waterways. Some of these, such as the Martesana, passed over other rivers, such as the Molgora and the Lambro. Ludovico il Moro, the client, was worried that a ship passing over this bridge-canal, by adding its weight would exceed the load-bearing capacity of the bridge. Leonardo clearly explains what happens 2.1. Sinking the Titanic We start this demonstration with a projection of some scenes about the sinking of the Titanic [7]. In the movie [8] Jack and Rose are clinging to the stern railing, and shortly before the sinking Jack says: Take a deep breath and hold it right before we go into the water. The ship will suck us down. Kick for the surface and keep kicking. Do not let go of my hand. We’re gonna make it, Rose. Trust me [9]. Is it true that there is a suction effect accompanying sinking ships? And if so, why? One of the theories that can be found on the Internet about this suction effect is that it is due to the air contained in the ship, that, while escaping, ‘lightens’ the water, which is thus unable to sustain lifeboats and shipwrecked people. This myth was also the subject of an investigation by the MythBusters, [10, 11] Adam Savage and Jamie Hyneman, who have not found any evidence of the effect, but using a ship which was too small for the purpose. Indeed, the suction effect does exist it is documented by many eyewitness accounts [12] and it is easily replicable. Just take a tall, narrow container, fill it with (possibly colored) water January 2019 The great weight of the boat passing along the river over the arch of the bridge does not increase the weight on the bridge itself because the boat weighs exactly the same amount as the quantity of water it displaces [13], 2 Phys. Educ. 54 (2019) 015025 Teaching physics by magic and several demonstrations for illustrating this effect are available [14, 15]. 2.2. Diver in a bottle The Titanic was carrying a lot of wealthy people, and there have been several expeditions targeted to retrieve these objects and valuables [16]. We can simulate a diving expedition in a bottle. We build a Cartesian diver using the cap of Bic pen and plasticine (Pongo) [17], we close the hole in the top of the cap and then we burden the spout so that the diver is barely floating. Then we introduce it in a plastic bottle filled with water to the brim, and we plug the bottle. How can we convince the diver to go down, given that the bottle is sealed? We show that we could induce the diver to go down and up at will, ‘magnetizing’ any object (or an attendee hand) with our ‘mental power’. Actually, what we do is just to increase the pressure inside the bottle by squeezing it with one hand (in order to not let attendees discover the trick, the bottle must be completely full otherwise its deformation is too evident). Some already prepared bottles are circulated among attendees so that they can check the effect by themselves. But now the question is: What happens inside the bottle when we squeeze it? OK, the internal pressure increases, and so what? Why is the diver sinking? Some proposed explanations concerning the increasing density of water, which, in any case, would rather favor the floating. It is convenient to ‘break’ the experiment in pieces and ask: why is the cap floating, even if it is made by material denser than water? Finally, the right interpretation arises: it is the air bubble inside of the cap that makes it float, and by increasing the pressure of the incompressible water, the volume of this bubble reduces, decreasing the buoyancy that keeps the diver afloat. This phenomenon is then shown visually, replacing the diver with one made with a transparent cap. Given that the water pressure increases with depth, by adjusting the strength of the hand is also possible to also let the diver remain stationary at a certain depth. Just a demonstration of the principle of Archimedes, coupled to Stevin’s (hydrostatic paradox) and Boyle’s laws [18, 19]. The show can be made January 2019 Figure 1. Lowering the sea level after the sinking of the Titanic. more dramatic by sinking a ‘precious necklaces’ made with the cap rim of disposable plastic bottles, some plastic beads and glue, and having them retrieved by the diver, which has some hooks attached to the plasticine [17]. 2.3. Water in the net This experiment usually is carried out with a piece of cloth (a ‘hanky’) [20, 21] but we use a wire gauze [22]. We stretch a piece of tulle over a glass jar and we secure it with a rubber band. We fill the jar through the tulle to show that the water can pass. Then we close the jar with a piece of plastic and turn everything upside down. Finally, we slowly side slip the plastic. We show that the water does not fall, unless one tilts the jar. The amazing thing is that one can repeat the trick with a net with a very broad mesh, with holes up to about half a centimeter in diameter. In this case the success heavily relies in keeping the surface horizontal. Just a small tilt or hit of the jar (or a fold in the mesh) and the water falls. In reality, one should be more surprised by the difficulty of keeping the water in the net. Indeed, we show that it is possible to use a straw with a closed upper end for ‘pipetting’ a water sample. One can use a straw with quite a large ‘hole’, up to one centimeter, and the water stays there. The difference is that the mesh is equivalent to many coupled straws (because they communicate through the liquid). To show what happens, we built a device with two coupled straws (figure 2), each with a diameter of about 6 mm. If the straws are half-filled, they are quite stable, and the device can be inclined without problems. However, if the device is filled up to the central straw, it is almost impossible to keep the water in. 3 Phys. Educ. 54 (2019) 015025 F Bagnoli et al Since the two water-air membranes are connected by water, each of them is transferring pressure to the other, similar to two connected soap bubbles [23]. Even a small inclination will increase the weight on the lower water–air surface, surpassing the surface tension. With a separate straw, one can show that the surface tension and the depression inside the straw can keep the water up to the level in the central arm of the ‘T ’ device, demonstrating that the leakage effect is due to the coupling and not just the water pressure. 2.4. Balancing broom Let us switch to mechanics. One almost magical ‘trick’ is the tightrope broom [24, 25]. We take a broom and we ask an audience member to find its center of mass, i.e. the point at which it can be balanced on one finger. Of course, to find this point one needs to make several attempts, and we point out that if the broom will fall unless it is perfectly balanced. After that, we blindfold the volunteer, ask him/her to hold his/her arms open and horizontal and we put the broom on the indices of the volunteer’s hands. Then we ask him/her to slowly approach his/her hands: with a great astonishment of the audience, the broom does not fall, and the two fingers join directly beneath its center of mass (figure 3). Why? The balance of a rigid body is given by the vanishing of the sum of forces (otherwise the body accelerates), but also by the fact that the sum of torsions (force times distance) must be zero, otherwise the body will rotate. The two fingers do not exert the same force: the finger closer to the center of mass exerts a larger force, as can be verified directly. Now the friction comes into play. The intensity of friction is given by the coefficient of friction (the same for the two fingers) and the pressing force. The finger closer to the center of mass exerts a larger force and therefore feels more friction. Hence, it is the farthest from the center of mass that slips, until it gets closer to the target than the other hand. So, the two fingers alternately slip until they join under the center of mass of the broom. Figure 2. Coupled straws. When the water level is below the connecting channel (left) the two straws are practically uncoupled and they can sustain a certain height of water (even much higher than in figure). When the straws are coupled by water (right) the difference in height causes the breakdown of the lower surface and the water drops away. Figure 3. Forces acting on the broom. high coefficient of restitution) for instance a basket and a tennis ball [26, 27]. We first illustrate their elastic properties, showing that they rebound to amost the starting height, and that, as pointed out by Galileo, all bodies fall with the same acceleration. This implies that the two balls, if dropped from the same height, hits the ground at the same time and with the same speed. We then stack them, with the light tennis ball on the top of the heavy basket ball. What height will the upper ball reach? The same as before? Twice? More? To the surprise of the audience, the tennis ball jumps 3–4 meters high, even more. Why? To get the answer in a simple manner (figure 4) we suppose that the balls are perfectly elastic bodies, and that the mass of the bigger one is infinitely greater than that of the other [28]. Attendees have to just remember one formula, that comes from the conservation of energy: that the height h0 is proportional to the square of the final velocity v2. 2.5. Bouncing balls Another magical effect can be produced with two balls inflated more than usual (so that they have a January 2019 4 Phys. Educ. 54 (2019) 015025 Teaching physics by magic Conservation of energy E0 = mgh0 = Ef = After the collision of the first ball Elastic collision and m2 << m1 1 mv2 2 and therefore h0 = v2 2 v2 = −v 2 ~ ∼ v 2' _ 2v 1 v1 = v 1 ~ ∼ v 1' _ 0 2g h0 In the external reference system In the reference system of the first ball Elastic collision −v v 2 ~ v2 = −2v 1 ~ v1 = 0 From which hf = (v'2) 2g 2 _ 3v v'2 ∼ 1 _v v'1 ∼ 2 = 9 v2 = 9h0 2g Figure 4. Approximate calculation of the collision of two perfectly elastic balls, with infinitely different masses. In an ideal world, the lighter ball should arrive at a height of... 9h0 ! If we drop the two balls from a height of one meter and a half, we can easily reach the ceiling (perhaps smashing a lamp, with a significant visual impact). If we put a third ball on the top of the second one, we could get to... 49h0 ! From a practical standpoint, it is not easy to keep the balls balanced before dropping them. We need to prepare some support with a plastic material such as clay to be inserted between them. Another possibility is to use crazy balls with a hole, using a stick to guide them [29]. The speaker can use this demonstration for illustrating the mechanism for the explosion of a supernova [28]. If one uses a basket and a tennis ball, it is also possible to complement the astronomical argument with an additional question: the relative size of these balls is roughly the same of that of the Earth and the Moon. At what distance should one place the tennis and the basket January 2019 ball to have the same scaling? And where should one place the International Space Station (ISS)? Attendees are surprised to discover that the Moon stays at 32 Earth diameters, i.e. about 8 meters, while the ISS is just one centimeter away from the basket ball. But if the ISS is so close, why are astronauts weightless? Is there no gravity in space ? [30, 31]. 2.6. Drinking bird At the beginning of each show we place in a corner a drinking bird (figure 5), which continues oscillating all the time. We regularly remind attendees to check this movement. At the end of the show, we ask our public to propose explanations of how this toy works. Where does it take the energy to move from? If it extracts heat from the environment, is not it violating the second law of thermodynamics according to Kelvin’s statement? 5 Phys. Educ. 54 (2019) 015025 F Bagnoli et al Evaporation of water (heat absorber) High pressure vapour Low pressure vapour High pressure vapour exiting Wet felt Heat source (environment) Figure 5. Drinking bird. There are many articles and videos on the drinking bird that illustrate the physics behind this toy [32–35]. Indeed, the drinking bird is a thermal machine which operates in a manner similar to the Italian ‘moka’ coffee machine (a device well-known by our audience). In both devices there is a boiler with a spout, fishing near the bottom. The heat vaporizes the liquid (water for the coffee maker, an ether for the bird), and the pressure increases, pushing the liquid into the exhaust pipe. In the moka, this tube is open and brings to the overlying coffee container. In the drinking bird, the tube brings to a closed overlying tank. When the liquid rises, the center of mass shifts, and the bird is jointed in such a way that at a certain point it tilts, the bird beak dips in the water and the liquid returns in the lower container. We point out that in the bird tube there is no air: the parts without the liquid are filled by its vapor. When the liquid rises, the steam in the upper tank must condense, and it does so because the pressure increases while the low temperature is maintained by the evaporation of the water (the upper tank is covered with a felt coating that is kept wet by the water ‘drawn’ by the beak). January 2019 So, the bird is a ‘conventional’ thermal machine. If, however, we include in the thermodynamic system also the glass with the water, something goes wrong: the whole system is at the same temperature (we show it with a thermometer), how can it work? The fact is that the air is not saturated with vapor (if it were so, the mechanism would stop), and therefore the water tends to evaporate, extracting heat from the bird. The motion is the result of a competition between energy and entropy, as in mixtures of ice and salt. If one closes the system inside some container (a transparent plastic box will do the job), it starts slowing down. Finally, the system reaches equilibrium, and the bird stops moving (as it does sometimes in sultry days) [36]. 2.7. Tea leaves/meandering rivers Fluids in motion are particularly suitable for magic tricks. This demonstration deals with a problem also faced by Einstein [37, 38]: why do tea leaves always gather at the center of the cup when one stirs the liquid [39]? The leaves are heavier than water and therefore should, by centrifugal force, 6 Phys. Educ. 54 (2019) 015025 Teaching physics by magic go towards the outer edge of the cup (as indeed they do at the beginning). So, where does the mysterious force which brings them back to the center, even in a cup with a convex bottom, come from? And what has this phenomenon to do with the fact that the rivers, in broad valleys, tend to become more tortuous with time (figure 6)? To show the first part of the experiment, we use a glass jar for preserves, with a convex base. We show that when the water is at rest, leaves on the bottom are resting along the edge, because it is the lowest place. We therefore set the water in rotation, and we point out that the surface of the water takes the form of a parabola, higher towards the outside, an easily explainable effect based on the centrifugal force. By projecting the jar seen from the bottom with the help of a webcam (and circulating the jar), all attendees are able to see that initially the tea leaves indeed continue to stay along the outer edge, but that shortly after, the ring formed by the leaves in rotation begins to shrink, until the leaves collapse into a central region, the highest point of the bottom of the jar [32]. The explanation (figure 7), allows us to introduce the flow in a bending river as if it were ‘half’ of the cup. We show some pictures (an example is shown in figure 6) illustrating how the rivers, if left free to determine their path, instead of going straight to the sea tend to become more and more tortuous, until they cross their own bed and ‘cut away’ a loop which forms a small lake in the shape of a horseshoe or a crescent. The same behavior is responsible for the fact that when it rains we see the formation of meandering water threads on car windows (figure 8). We reproduce this behavior using a plastic plate tilted a few degrees relative to the vertical (resting in the bowl) and an infusion tube. At first, we drop some water droplets, and we show that they come down along the direction of maximum slope. Then we let the water run continuously without moving the tube and we see that the water thread tends to displace in a side direction, making increasingly accentuated curves (figure 8). The effect is more noticeable the faster the water flow. The explanation that proves the existence of this secondary flow, makes it possible to speak also of the physics of the atmosphere, namely the formation of tornadoes [38]. January 2019 Figure 6. Meanders of Rio Negro river, Patagonia, Argentina seen from the ISS space station. Image courtesy of the Earth Science and Remote Sensing Unit, NASA Johnson Space Center. NASA Photo id ISS022-E-19513 from http://eol.jsc.nasa.gov. 2.8. The obedient balloon For another demonstration concerning moving fluids, we exploit the famous experiment of an air balloon suspended above the jet of an air dryer, or a ping-pong ball above one blowing into a straw, and the raise of a strip of paper by blowing above it with a straw. The are many videos [40] and articles [41] for this topic. For the balloon we call a volunteer from the audience, we blindfold him/her, turn on the dryer and show that a light ball or a balloon stays in equilibrium without targeted efforts. One can also carry it around as if it were connected by a wire. We can do more: the balloon remains stationary even if it the jet is not on vertical, up to considerable inclinations (figure 9). Why? We want to avoid explanations based on Bernoulli’s law, whose application is problematic [42, 43]. We always use a microscopic model of the fluid [44], visualized using ping-pong balls. We all know that a gas is composed of molecules traveling at a speed close to that of sound, and that the pressure is given by the impact of such molecules on a surface. Actually, one does not need a real surface: the pressure is also given by the number of particles crossing an ideal surface per unit of time, multiplied by their momentum. 7 Phys. Educ. 54 (2019) 015025 F Bagnoli et al In a gas at rest, the molecules are traveling in all directions so that a suspended sheet of paper remains stationary not because there are no collisions, but because it receives pushes equally on both of its surfaces. But if we force a gas to travel in one direction, there are fewer molecules passing through a direct ideal surface parallel to the flow, with respect to a gas at rest. In contrast, if the surface is now perpendicular to the flow, there are more molecules traversing it from the side where the flow impacts than in the opposite direction. Thus, a moving fluid exerts a lower pressure, compared to the same fluid at rest, in the direction perpendicular to that of the flow, while obviously exerts a greater pressure in the flow direction (figure 10). Therefore, the balloon, which is placed in a region in which the flow widens, feels a transverse pressure that brings it back to the region in which the flow is faster. It is in equilibrium because it receives a transverse force and a direct one from the flow, which compensates for the force of gravity, and this also works up to a certain inclination. The molecular model allows one to easily introduce the Coanda effect, as due to intermolecular (Van der Waals) interactions. z Ug = mgz Uc = ½ mω 2 r2 Ug + Uc = cost fc = mω 2 r Transverse flux fg = mg Resulting force r Tea leaves Figure 7. Secondary (transverse) flow in a cup. 2.9. Roberto Carlos (Magnus effect) Figure 8. Rivulets of water on glass. Image courtesy of Markus Spiske. The difference in pressure (‘Bernoulli’ effect) or rather the viscous drag is also responsible for the ‘curved trajectory’ (Magnus effect) exploited in many sports such as baseball, soccer and table tennis [45, 46]. In our demonstration we show the effect using a cylinder built by rolling an A4 sheet and securing it with the adhesive tape, and a plastic slab [47]. Holding the slab slightly inclined with respect to the horizontal, we first drop a tennis ball, and marked the landing point on the ground. Then we show the cylinder, stressing that it is more lightweight than the ball, asking to vote on where it will fall with respect to the mark. Same place? Closer? Farther away? To the surprise of many, the cylinder even turns back. We also show a video of the famous goal by Roberto Carlos in 1997 [48], which always awakes the soccer fans component of the public, and a video of a basketball dropped from the top of a dam, with and without rotation [46]. January 2019 ω Figure 9. Balloon balanced on the jet of a dryer. The explanation is the standard one, based on the different velocities of the fluid in the reference frame of the ball. 8 Phys. Educ. 54 (2019) 015025 Teaching physics by magic 2.10.Vortex cannon/smoke rings Our last demonstration is with ‘real’ special effects, using a vortex cannon [49] made by a plastic bin with a round hole, closed by a rubber membrane (but one can use a simple cardboard box) where air is fired by hitting the membrane. We build a target consisting of a pyramid of plastic cups, and we use the cannon to break it, even from a distance. What kind of bullets does the cannon fire? What is their shape? How can an air ‘bullet’ get that far [50]? To show what happens, we use a smoke machine like those used for rock concerts. In this way one can ‘see’ the swirl donut-shaped (smoke ring) that comes out and travels throughout the theatre (it is convenient to use a cross-light source, on a black background) [51]. But why are smoke rings so stable? Again, the explanation involves the ‘Bernoulli’ effect: the smoke donut surface moves out from the hole of the donut itself, widening and then compressing, up to falling again into the hole. But inside the donut the air is still, as it is outside. So, there is a transverse pressure which confines the flow, and therefore the donut remains stable until the smoke keeps moving (figure 11). Figure 10. ‘Bernoulli’ effect. As said, the lessons have been organized as a magic show, and in fact, as a ‘result’, FB was invited on May 13, 2017 to participate in a real ‘magic’ show at the School of Magic ‘Corte dei Miracoli’ (Court of Miracles) in Livorno [57]. The difference between a magic show and a physics lesson is not in the demonstrations (which must always have some amazing aspect) but in the fact that one is going to ‘explain’ such effects. The explanation, we discovered, can be more surprising than the demonstration itself. Each lecture was attended by two or three classes, with a variable number of students between 30 and 50. The amplification system (sometimes present) was never used, since that would have required a headset wireless microphone for keeping hands free. 3. Experiences 3.1. Bocconi University and Free Age University These demonstrations have been first developed in 2014 for a series of lessons for the ‘Università dell’Età Libera’ (University of Free Age), Municipality of Florence [52] and at the Bocconi University (Milan), for the ‘sapere a tutto campo’ (knowledge across the board) special course [53]. Both courses have been repeated the following year upon request. All courses have been attended by about 40 people. 3.2.1. The evaluation questionnaire. We developed a questionnaire to gather some feedback (see for instance [58]). The survey received 223 responses, collected during a period of three months in winter 2017, after an interval of at least one week after the lesson. The age of the respondents is evenly distributed from 15 years old to 20, as well as with regard to the gender. The experience was considered very positively (figure 12) and most of the topics have been further deepened in class or talking with peers (figure 13). For what concerns the various demonstrations (table 1), we can see (figure 14) that those better remembered were the obedient balloon, the gun vortex and bouncing balls while in the 3.2. High school lessons In the winter 2016/2017 one of the authors (FB) held 20 lectures in various schools in Tuscany for the Pianeta Galileo project of the Tuscany Region [54, 55], repeated the following year in other eight schools. The lessons lasted about one hour and a half. The videos of the first cycle of lessons are available on YouTube, and in the FisicaX blog [56]. January 2019 9 Phys. Educ. 54 (2019) 015025 F Bagnoli et al Figure 11. Vortex cannon and cross-section of a smoke ring. 125 100 75 50 25 0 Highly Quite So and so Little Nothing Figure 12. Reception. last places there are the dipping bird and the tea leaves, which evidently proves to be more difficult to explain or to understand. Surprisingly, the diver was fourth place. Pupils said they had redone some experiments (figure 15), preferably those that required less preparation. About half of the students have seen or shown the videos of the lectures or consulted the FisicaX site (figure 16). Finally, with regard to the negative aspects, most students denounced the audio or vision problems and expressed their appreciation for the use of the webcam to project the demonstrations on the screen. January 2019 3.3. The FoCoSTEP experience These demonstrations have also been the core of the didactical physics course of the FoCoSTEP program [58] in the 2017. FoCoSTEP is a new training program that has been conceived and created at the ESPE of Reunion (France). ESPE are state universities, i.e. graduate schools of education. The goal of the FoCoSTEP is to increase the quality and the quantity of science and technology taught in primary and nursery schools. The specificity of this training program is that it is based on the mutual and global collaboration between the three 10 Phys. Educ. 54 (2019) 015025 Teaching physics by magic 100 60 40 20 0 Yes, lesson Yes, Yes, classmates internet/ book Yes, friends Yes, family No Figure 13. Deepening. 100 I did not like and I do not remember I remember but I di not like it 75 So and so I liked but I do not remember 50 I liked and I remember 25 in g ci ng s ba lls D Ba iver sk et ba ll W at Bro er om in th en et Ti ta D ni ip c pi ng bi rd Te al ea ve s lo Bo un ok Sm Ba er on 0 Figure 14. Rating experiments. of interns in his class and supervises their work. Usually, couples are formed by a student with a scientific background and one with a humanistic one. Students are on stage one week every two. The week in which they are not on stage, FoCoSTEP students come to ESPE to follow courses and other activities. The FoCoSTEP professors go to schools to attend the lessons held by categories of the program’s actors: the graduate students qualifying as primary and nursery school teachers, the confirmed school teachers who hosts these students for an internship, and ESPE science and technology professors. During the school year, FoCoSTEP students do an internship in a Reunion primary school. Each confirmed school teacher hosts a couple January 2019 11 Phys. Educ. 54 (2019) 015025 F Bagnoli et al 80 60 40 20 in er at W Sm ok er in gs th e Te net al ea ve s Ti t a D ni ip c pi ng bi rd A vi de o N ot hi ng lls ba n Bo un ci ng r lo o iv e Ba D Br oo m 0 Figure 15. Experiments repeated by students. 2017 [60] and during the LivingKnowledge 2018 congress [61, 62]. students. Typically, each intern is ‘visited’ twice, once a semester. Most of the time the lesson is video recorded. At the end of the visit, the FoCoSTEP professor, the confirmed teacher in charge of the class and the two interns (the one who was ‘visited’ and his partner), debrief and make a critical analysis of the lesson that has just taken place. Video recordings will be used during sessions of ‘critical analysis of teaching practices’, to which all students and trainers attend. The physics program proposed to the FoCoSTEP graduate students is based on the physics demonstrations presented here. Students are strongly encouraged to reproduce (some of) these demonstrations during the classes they give to their internship schools. The results at the competitive examination needed to become a French school teacher encourage us to develop and extend this program into the coming school years. Indeed, the success rate of graduate students enrolled in the FoCoSTEP program is much higher than that of other students (90% versus 70%) [59]. 4. Conclusions In this paper we presented ten demonstrations to be used in a scholastic or recreational context with the purpose to teach physics in a more profitable way. This series of demonstrations, possibly expanded, could become a kind of laboratory, where students, after watching an experiment, are encouraged to propose explanations and discuss among themselves whether the proposed hypotheses can work, if they do violate any law physics, in the spirit of the peer instruction. As one can see in the videos [63], we tried to stimulate the discussion also using the trick to have students ‘vote’ the most plausible explanation, but obviously one cannot do much more in just one lesson, which should therefore be considered only a stimulus. Thanks to the two experiences described, with the help of the questionnaire and the results of the exams, we can say that a teaching method like the one described above definitely promotes a better learning of physics. Obviously (and unfortunately) it is not always possible to transform the teaching of physics into a magic show, but it certainly would not be bad if 3.4. Science shows All these demonstrations, plus others, have also been exploited by some of the authors (FB and GP) during physics shows, in particular Scienzestate January 2019 12 Phys. Educ. 54 (2019) 015025 Teaching physics by magic 100 75 50 25 0 I watched the video I showed the video I consulted the site What is FisicaX? n.d. Figure 16. Visit to the FisicaX [56] website. the teachers learned at least some element of theatricality and of the construction of a narration. Perhaps, we would not be so short of graduates in science and engineering. 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Budapest, Hungry) (www.livingknowledge.org/lk8/) [62] Bagnoli F and Pacini G 2018 Magic physics show at LK8 FisicaX (http:// fisicax.complexworld.net/video/ magic-physics-at-lk8) January 2019 Franco Bagnoli is a theoretical matter physicists working on the physics of complex systems in the University of Florence. He teaches basic and computational physics. He is the president of the association for the popularization of science and technology "Caffè-Scienza Firenze". Alessio Guarino is an experimental physicist working on fluid and fracture dynamics, social physics and education science. After obtaining a PhD in Physics at the École Normale Supérieure de Lyon, he was a researcher at the University of California at Santa Barbara (USA) for nearly two years. Back in France in 2001, he worked as a physics professor at the École Normale Supérieure de Lyon, the University of French Polynesia, the University of Marseille and the University of Reunion. Between 2011 and 2015 he headed, for the government of French Polynesia, the Department of Evaluation, Foresight and Performance [DEPP] of French Polynesia. He teaches physics at the Ecole Supérieure du Professorat et de l'Education (ESPE), University of La Réunion. Giovanna Pacini is a science communicator and the coordinator of the Florence Science Shop of the University of Florence. She teaches communication of science to PhD students of the University of Florence. 15 Phys. Educ. 54 (2019) 015025