### Normal stress and strain

Stress express the strength of material per unit of area. if an axial force in tension (P) subjected to a prismatic bar of cross-sectional area (A). prismatic means a member of the same cross-section through the length. in this case, the stress can be calculated using the following equation

the figure below shows a prismatic bar subjected to a tension force P. in figure B depicts the bar before being subjected to force. we can notice the original length of bar equal L. after subjecting the bar to force P the bar length increased. this increased in length induced by stress imposed in the bar. the change in bar dimension known as strain. tension force will cause an increase in bar length in the contrary the compression force will reduce the length of the bar. when the force applied at perpendicular to cut surface as shown in figure d this will be known as normal stress. applying the force at the center will result in a uniform distribution of forces as shown in d.
The principle of stress and strain are used everywhere. construction material such as steel and concrete should have a define stress value. so the designer will not exceed this value and design a safe structure.