A light rod of length 2m is pivoted at its centre and two masses of 5kg and 2kg are hung from the ends as shown in figure.  The initial angular acceleration  of the rod assuming that it was horizontal in the beginning is $\alpha =$  ____________$(\mathrm{rad}/{\mathrm{s}}^2)$. Take $\mathrm{g}=9.8\mathrm{m}/{\mathrm{s}}^2$.