Strain and stress in continuous media

When no external force is applied to a solid object, and it is in equilibrium, it has some particular shape. When forces are applied, its shape changes. This effect, referred to as strain, will be defined precisely in section 1.2. For a segment of material of the object, forces acting upon it may originate outside the object, or may come from adjacent segments of the material. Those internal forces that act on the surface of the segment constitute the so-called stress, which will be precisely defined in section 1.3. Stresses produce strain; the relationship between the two will be discussed in section 1.4. Equilibrium will be discussed in section 1.5.