**Principles of assembly**

Fundamental to structural design is the study of mechanics: the action of a force on a mass. The study of statics is that branch of mechanics which studies the forces that hold a mass in a state of equilibrium. A force is an “agency,” which tends to change the condition of rest of a rigid body; such agencies can be considered as push or pull forces, which do not produce motion. In short, statics is the structural design of motionless objects.

Newton’s Third Law of Motion (described in his *Principia Mathematica* *Philosophiae Naturalis* of 1687) is the law of action and reaction:

**Whenever a particle A exerts a force on another particle B, B simultaneously exerts a force on A with the same magnitude in the opposite direction. ...**

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