In the first part of the master thesis a review of the annual reports of the National examinations centre for all the years, in which the knowledge of physics was assessed, is presented. We collect problems related to buoyancy and pressure and distribute them according to the difficulty classes they are classified in. We are focusing on the problems, which seem to be the most difficult for the pupils. From the analysis of the results we conclude how much knowledge about these topics pupils gain in the primary school. In the main part of the master thesis we analyse in detail two problems, related to buoyancy and pressure that were included in the National assessment of knowledge of physics in the year 2012. We design classes into which we distribute answers of a few hundred pupils. From the frequencies of typical incorrect answers we try to understand why these pupils did not solve the problem correctly. In the problem related to the understanding of buoyancy, most of the pupils started the wrong way from the very beginning. Instead of calculating the force of buoyancy, pupils calculated some number using a random combination of data provided and simple arithmetic operations. At the end they usually added to the numerical result the correct unite for force – the newton. We have also found out, that the majority of pupils do not write correctly the condition for the equilibrium of forces. In the problem related to the understanding of pressure, most of the pupils started correctly, but only a few obtained the correct final result. In most of the cases the reason for that was that pupils used the wrong surface in the calculation. An important finding is also that a lot of pupils equate pressure and force, which we deduce from the incorrect use of units. This indicates a lack of understanding of the concept of pressure.
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