A film of the liquid is supported in a vertical rectangular area, the top border of which is a sliding wire. An external  force of  5 mN acts in vertical upward direction on the sliding wire of length 50 mm and linear mass density of   $1.75\times {10}^{-3}kg/m$. Find the surface tension of the liquid in (N/m) (g =10 $m/s^2$)