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The bottle flip challenge demystified: where is the centre of mass?

Bodies can have very weird trajectories when tossed into the air. The amazing motion of a water bottle flip is qualitatively described as related to the position of the centre of mass (CM) with respect to the body’s frame of reference. Although this is rather easy to guess from simple observations, it is not so obvious to verify quantitatively with the naked eye. In this work, we use video analysis and a computational model to show that the position of the CM changes in the frame of reference of the bottle, while it describes a parabolic trajectory in the frame of reference of the laboratory. We also show that the behaviour of the CM also explains the weird motion of the bottle. It can be observed graphically and discussed in the classroom with students, as it only needs some critical thinking and practically no math.