Balance contd
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The Rolling Clown
This traditional wooden toy consists of a clown figure which somersaults along a pair of parallel bars when given a push. Its appeal lies in the fact that it rolls relatively slowly and there is some doubt over whether he will complete one more somersault before swinging back to the upright position.
The Physics
The figure is stable in the upright position but the Centre of Gravity is only just below the axle, the point of suspension. There is therefore only a very small restoring couple when it is displaced.
As it rolls the centre of gravity rises and falls. Before each roll there must be sufficient kinetic energy to raise the centre of gravity of the figure above the pivot.
The figure has a relatively large ‘moment of inertia’ so even at slow speeds there is sufficient kinetic energy of rotation to provide the required gravitational potential energy to lift the weight.
Rolling friction gradually dissipates the initial kinetic energy until there is insufficient to complete another forward roll. The figure then oscillates, pendulum-like, until it comes to rest upright.