A plank of mass `m_1` with a bar of mass `m_2` placed on it lies on a smooth horizontal plane. A horizontal force growing with time t as `F^'=at` (a is constant) is applied to the bar. Find how the accelerations of the plank `w_1` and of the bar `w_2` depend on t, if the coefficient of friction between the plank and the bar is equal to k. Draw the approximate plots of these dependences.