Two masses m are attached to opposite sides of a rigid rotating shaft in the vertical plane. Another pair of equal masses $m_1$ is attached to the opposite sides of the shaft in the vertical plane as shown in figure. Consider $m=1 \: kg$, $e=50 \: mm$, $e_1=20 \: mm$, $b=0.3 \: m$, $a=2 \: m$ and $a_1 = 2.5 \: m$. For the system to be dynamically balanced, $m_1$ should be ________ $kg$.
