A $0.20\,\text{kg}$ block and a $0.50\,\text{kg}$ block are joined by a light inextensible string which runs over a frictionless pulley. At the start, the blocks are kept stationary. The $0.20\,\text{kg}$ block lies on a rough horizontal surface, whereas the $0.50\,\text{kg}$ block hangs freely in air. Air resistance is negligible. Once the blocks are released, their acceleration has magnitude $2.0\,\text{m s}^{-2}$. What is the magnitude of the frictional force between the $0.20\,\text{kg}$ block and the rough surface?
- A$3.5\,\text{N}$
- B$3.9\,\text{N}$
- C$4.5\,\text{N}$
- D$6.3\,\text{N}$