"StressWhen a stretching force (tensile force) is applied to an object, it willextend. We can draw its force - extension graph to show how it willextend. Note: that this graph is true only for the object for which it wasexperimentally obtained. We cannot use it to deduce the behaviour ofanother object even if it is made of the same material. This is becauseextension of an object is not only dependent on the material but also onother factors like dimensions of the object (e.g. length, thickness etc.) Itis therefore more useful to find out about the characteristic extensionproperty of the material itself. This can be done if we draw a graph inwhich deformation is independent of dimensions of the object under test.This kind of graph is called stress- strain curve.StressStress is defined as the force per unit area of a material.i.e. Stress = force / cross sectional area: where,s = stress,F = force applied, andA= cross sectional area of the object.-2 Units of Strain: Nm or Pa.Strain is defined as extension per unit length.Strain = extension / original lengthwhere,e = strain,l = the original lengtho e = extension = (l-l ), ando l = stretched lengthStrain has no units because it is a ratio of lengths. We can use theabove definitions of stress and strain for forces causing tension orcompression. If we apply tensile force we have tensile stress and tensile strain. If we apply compressive force wehave compressive stress and compressive strain. A useful tip: Incalculations stress expressed in Pa is usually a very large number andstrain is usually a very small number. If it comes out much differentthen, you've done it wrong! "