Two blocks, of mass 0.20 kg and 0.50 kg, are connected by a light inextensible string that passes over a frictionless pulley. [Figure 7.1] The blocks are initially held stationary. The block of mass 0.20 kg rests on a rough horizontal surface. The block of mass 0.50 kg is suspended in air. Air resistance is negligible. When the blocks are released, they have an acceleration of magnitude 2.0 m s⁻². What is the magnitude of the frictional force between the block of mass 0.20 kg and the rough surface?