Define Newton's first law of motion. Why is it called the law of Inertia?

Write an expression for the change in momentum of a body of mass m moving with velocity $\mathrm{v}$if  $\mathrm{v}<<\mathrm{c}$  ,where $\mathrm{c}$ is the speed of light.

Write an expression for the change in momentum of a body of mass$\mathrm{m}$ moving with velocity $\mathrm{v}$ if $\mathrm{v}\to \mathrm{c}$.