In the figure shown, a solid sphere of mass ‘m’ and radius r is released from a height 6r to slide down a smooth surface. A plank of same mass ‘m’ touches the horizontal portion of the surface at the ground. The co-efficient of friction between the plank and the sphere is mu and that between the plank and the ground is mu//4. Find the work done by the friction force between the plank and the ground till the sphere starts pure rolling on the plank. Neglect the height of the plank.