Statics: Equilibrium

A plate-bottle system

This model demonstration shows how an equilibrium state can be achieved.

(a)                                                                               (b)
Fig. 1-5: Equilibrium of a bottle and wood system

Fig. 1-4 shows that the ruler is in stable equilibrium when it has two round pen supports and becomes unstable equilibrium when it has only one round pen support. This however does not mean that a body placed on a single support cannot achieve a state of stable equilibrium.

Figure 1.5a shows a bottle of wine and a piece of wood with a hole. The bottle can be supported by the wood when the neck of the bottle is inserted into the hole to the maximum extent, and the two form a single wood-bottle system in equilibrium as shown in Fig. 1-5b.

The wood-bottle system, supported on the narrow wooden edge, is and feels very stable, because:

  • The two external forces from the weights of the bottle and the wood are equal to the reaction force generated from the table;
  • The sum of the moments of the two action forces about the point where the support force acts are equal to zero.

Another way of saying this is that the centre of mass of the wood and bottle system lies over the base of the wood.