A force F at an angle 0 above the horizontal is used to pull a heavy suitcase of weight mg a distance d along a level floor at constant velocity. The coefficient of friction between the floor and the suitcase is u. The work done by the frictional force is: (A) -Fd cos 0 (B) - u Fd cos 0 (C) -umgd (D) -u mgd cos 0 Answer A Answer B Answer C Answer D