A man uses a rope to haul a box of weight 600 N up an inclined wooden plank of effective length 3.0 m and onto a platform 1.0 m high. If in doing so, he exerts a force of 400 N on the rope, calculate (1) the frictional force between the case and the plank; the velocity ratio of the machine; the mechanical advantage; (iv) the useful work done in joules. ​