A horizontal plane supports a plank with a bar of mass `m=1.0kg` placed on it and attached by a light elastic non-deformed cord of length `l_0=40cm` to a point O(figure). The coefficient of friction between the bar and the plank equals `k=0.20`. The plank is slowly shifted to the right until the bar starts sliding over it. It occurs at the moment when the cord deviates from the vertical by an angle `theta=30^@`. Find the work that has been performed by that moment by the friction force acting on the bar in the reference frame fixed to the plane.

Answer

`A=(kmgl_(0))/(2)(1-cos 0)/((sin0+k cos0)cos0)]`