Stable, metastable and unstable states a simple analogy

1 Stable, metastable and unstable states a simple analogy. A simple mechanical model is shown in Fig. 2.37 a block on a stand may be in different equilibrium states. In A and C the centre of gravity (G) of the block is lower than 

Position C does not correspond to the lowest minimum of the energy following a small displacement, the block will return to the initial position whereas large displacements will move the block to the more stable position A. In A there is an (absolutely) stable equilibrium and in C a metastable equilibrium. For this mechanical system the stability conditions and the trends of spontaneous (natural) processes are related to minima (relative or absolute) of the gravitational potential energy. 

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