Stresses and Strains:

While we apply forces on solids, deformations are generates if the solid is avoided from motion (along with acceleration = force/mass of solid) either partially or fully. If the solid is not restrained, it might undergo displacements without change in size or shape and these displacements are termed as rigid body displacements. If the solid is restrained through some other force, called as reaction, that keeps the solid in equilibrium, the force will be transmitted by the medium of the solid to the restraining support.

Now let consider the solid shown in Figure (a) that is pulled through the force P at the right hand side and restrained through a support at the left hand side end. If the solid is to remain within static equilibrium we know in which the support should exert an equivalent pulling force within the direction opposite to that of force P. This restraining force, termed as support reaction, is display as R in Figure (a). Now let consider the solid being divided within a number of small bits or elements of various lengths as display in Figure (b). A force P applied at the RHS end of component 1 is transmitted to the support by the medium of elements 2, 3 and 4. The similar thing is true even if you separated the solid within hundreds or even millions of elements.

Figure

Now, to remain the element 1 in static equilibrium and element 2 should exert a force R (= P) on element 1. At the similar time, element 1 exerts a force P on element 2, so in which equilibrium exists at the point or surface of contact. The element 2 is again pulled through the equilibrant R exerted through element 3. Therefore equilibrium exists at each and every point or surface of contact and each of the elements are kept in equilibrium.