1 joule is equal to ______________erg. 

We know

 1 joule  $=1{\text{\hspace{0.17em}kg\hspace{0.17em}m}}^{\text{2}}{\text{s}}^{-2}$

$={10}^3\text{g\hspace{0.17em}}\times {\left({10}^2\text{cm}\right)}^2{\text{s}}^{-2}$                $\because 1\text{kg\hspace{0.17em}}={10}^3\text{g}$

${\text{1m\hspace{0.17em}=\hspace{0.17em}10}}^2\text{cm}$

$\Rightarrow {10}^3\text{g\hspace{0.17em}}\times {10}^4{\text{cm}}^{\text{2}}\times {\text{\hspace{0.17em}s}}^{-2}$      

 $(\text{Since\hspace{0.17em}}1{\text{gcm}}^{\text{2}}{\text{s}}^{-2}=1\text{\hspace{0.17em}erg\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}i.e.\hspace{0.17em}unit\hspace{0.17em}of\hspace{0.17em}energy\hspace{0.17em}in\hspace{0.17em}C.G.S\hspace{0.17em}system)}$