A person carries a plank of wood 1.8 m long with one hand pushing down on it at one end with a force F1 and the other hand holding it up at 55 cm from the end of the plank with force F2. If the plank has a mass of 18 kg and its center of gravity is at the middle of the plank, what are the magnitudes of the forces F1 and F2?

Which of the following is part of the explanation or description of electromagnetic waves? (select all that apply) a. electrically charged molecules in a medium transfer their energy to nearby electrically charged molecules. b. a changing electric field causes a changing magnetic field. c. magnetic molecules in a medium transfer their energy to nearby magnetic molecules. d. a changing magnetic field causes a changing electric field.