That which tends to cause change. It is not possible for a force to occur singly. Every force is opposed by an equal and opposite force, e.g., gravitational, inertia, friction. Gravity is really an inertia force, given by the product of the mass and the gravitational acceleration. It is referred to as the "weight" of an object.

Elastic forces are important in bridge design, as every part of the bridge moves elastically to provide forces opposing those produced by the loading . Bridges are static affairs, so inertia forces are not normally of prime interest. Dynamic loading, however, may involve consideration of such forces, as does vibration.

When the forces are known together with the area over which they act, the stresses and strains in the material can be considered. If limiting values of stress and strain are known , then for given forces, the areas can be determined.

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