12. A 20.0-kg sled is being pulled across a rough horizontal surface at a constant velocity. The pulling force has a magnitude of 80.0 N and is directed at an angle of 30 above the horizontal. a) (4 points) Calculate the work done by the pulling force. b) (3 points) Calculate the work done by the normal and gravitational forces. c) (4 points) Calculate the change in the internal energy of the system. d) (3 points) Calculate the coefficient of kinetic friction.