Wind is blowing steadily at a velocity of $12\frac{m}{s},$ where a wind turbine with a blade diameter of $50m$ is used for electric power generation. Calculate the mechanical energy of air per unit mass and the power generation potential of the wind turbine. Calculate the efficiency of the wind turbine$-$generator system if the actual electric power generation is $780kW.{}_{\text{a}\text{i}\text{r}}=1\mathrm{.}25k\frac{g}{m^3}$