What is strain? Give its units. Define longitudinal stress.

(N/A) Strain is defined as the ratio of the change in dimension to the original dimension of a body when it is subjected to a deforming force. Mathematically,$\text{Strain} = \frac{\Delta L}{L}$.
Since strain is a ratio of two similar physical quantities (lengths),it is a dimensionless quantity and has no units.
Longitudinal stress is defined as the restoring force per unit area when the deforming force produces a change in the length of the body. It is given by $\sigma = \frac{F}{A}$,where $F$ is the applied force and $A$ is the cross-sectional area. Its $SI$ unit is $\text{N/m}^2$ or $\text{Pascal (Pa)}$.