The fundamentals of classical mechanics are the foundation of more complicated and ingenious
physical theories. Therefore, an educational experiment revealing to students the physical nature of
mechanical phenomena is important. The effectiveness of this experiment manifoldly increases if
students not only perform it by themselves, but also make the necessary appliances with their own
hands. Such devices should be modern in order to elicit the students’ interest, and very simple so
that creating them does not require much time and effort. This paper proposes a device that can be
used in the experimental study of the relativity of mechanical motion and in the investigation of
the motion of the solid mass centre in the Earth’s gravitational field. The device is a soft disc
into which a power supply, a switch, a pulse generator and two coloured light emitting diodes are
fixed, so that the mass centre of the device does not coincide with the geometrical centre of the
disc. One of the…