A person pulls her 70 N bag 200 meters at a constant speed. The force exerted is 40 N at 50 degrees. I can figure out the work the person does (5142 J). But I'm having trouble figuring out the 'work done by the force of friction' I know that Wfriction = delta KE + delta PE ...Since the speed is constant KE should be 0, but I'm not sure what to do about PE.. I know PE = mgy and delta PE = PEfinal - PEinitial but since it's a horizontal surface wouldn't that be 0? I must be doing something wrong because it can't be 0..