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Rigid disc (representing the main transversal section of the turtle shell) rolling on a horizontal line. The thick dashed line denotes the orbit of the centre of gravity (G) during rolling. The peaks/valleys of the orbit correspond to local extrema of the potential energy, i.e. unstable/stable equilibria of the rigid body. For convex, homogenous two-dimensional discs, there are always at least two stable points (Domokos et al. 1994).
