A slider crank mechanism has slider mass of $10$ kg, stroke of $0.2$ m and rotates with a uniform angular velocity of $10$ rad/s. The primary inertia forces of the slider are partially balanced by a revolving mass of $6$ kg at the crank, placed at a distance equal to crank radius. Neglect the mass of connecting rod and crank. When the crank angle (with respect to slider axis) is $30^\circ$, the unbalanced force (in newton) normal to the slider axis is _____