a person carries a plank of wood 2.00 m long with one hand pushing down on it at one end with a force f1 and the other hand holding it up with force f2 at 0.500 m from the end of the plank where f1 is applied. if the plank has a mass of 20.0 kg and its center of mass is at the middle of the plank, what are the magnitudes of forces f1 and f2?