draw the fbd n observer the forces, the plank will apply 5N force (kinetic friction) on the block in backward direction n by newtons 3rd law an equal n opposite force will be applied on the plank i.e. 5N.
Now calculate their respective accelerations n this has been transformed to a problem of kinematics,where the block has acceleration of 5m/s2 in backward direction n velocity of 10m/s in forward direction and plank will have acceleration of 2.5 m/s2 in forward direction.Find the time when the block n plank have same velocity,then they will be at rest w.r.t eachother n friction will stop acting.
