Two blocks of masses $m_1=1kg$ and $m_2=2kg$ are connected by a non-deformed light spring. They are lying on a rough horizontal surface. The coefficient of friction between the blocks and the surface is $0.4$ . What minimum constant force $F$ has to be applied in horizontal direction to the block of mass $m_1$  in order to shift the other block $\left(g=10m{s}^{-2}\right)$