Explain the difference between engineering strain (or stress) and true strain (or stress). Derive the expression for how true strain is related to engineering strain. (You can find the result in Section 6.7 of Callister, 10th edition). Also, state the relationship for how true stress is related to engineering stress (you do not need to derive this one).

$$\begin{gathered} \mathrm{let} \mathrm{consider} \mathrm{a} \mathrm{specimen} \\ {\mathrm{A}}_{\mathrm{i}}=\mathrm{initial} \mathrm{cross} \mathrm{section} \mathrm{area} \mathrm{of} \mathrm{the} \mathrm{specimen} \mathrm{load} \mathrm{is} \mathrm{not} \mathrm{applied} \\ {\mathrm{L}}_{\mathrm{i}}=\mathrm{initial} \mathrm{length} \mathrm{of} \mathrm{the} \mathrm{specimen} \mathrm{when} \mathrm{load} \mathrm{is} \mathrm{not} \mathrm{applied} \\ \mathrm{P}= \mathrm{load} \mathrm{apllied} \\ {\mathrm{A}}_{\mathrm{a}}= \mathrm{actual} \mathrm{cross} \mathrm{section} \mathrm{area} \mathrm{of} \mathrm{the} \mathrm{specimen} \mathrm{when} \mathrm{loaded} \\ {\mathrm{L}}_{\mathrm{a}}=\mathrm{actual} \mathrm{length} \mathrm{of} \mathrm{the} \mathrm{specimen} \mathrm{when} \mathrm{loaded} \end{gathered}$$