The concept of the centre of mass of a rigid body as a virtual point where the weight force acts is not easy to understand without a number of supporting school experiments. In school practice, however, experiments on this topic are often limited to a few of the simplest cases in which a simple flat body, such as a triangle or rectangle, is hung in two or mostly three directions to show where the corresponding plumb lines intersect. Typically, simple wooden bodies are used, on which the plumb lines are already drawn through the centre of mass. However, such experiments can be boring for students and are probably insufficient to illuminate all aspects of the topic. Furthermore, if the experiments are only demonstrated by the teacher rather than being performed in groups, the opportunity to train students' skills and develop nature-science competences is missed. We therefore prepared and performed a series of group experiments in logical sequence for students of the 8th and 9th grades of primary school, so that their full active participation was invoked. The experience with such an experiment setup with very simple equipment, together with the open discussion of results, increased pupil motivation for physics and perhaps also improved understanding of some physics problems regarding the centre of mass, even for younger students.