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As part of a test, participants were asked to give a short explanation for the fact that in free fall and in vacuum all objects experience the same acceleration. The test was given after most of the students had seen the dropping-tube-experiment and had listened to a proper explanation. The analysis of all collected answers reveals a series of common misunderstanding or lack of knowledge about some basic concepts in mechanics, like inertia, gravity, air resistance, property of vacuum, etc. the traditional classroom activity, based on experiments (dropping-tube) and mathematical derivation (mg=ma) does not seem to be sufficient for the majority of our students to reach a sufficient level of understanding. Possibilities for improvements are discussed.