1. The problem statement, all variables and given/known data If you walk up a flight of stairs at constant speed, gaining vertical height h, the work done on you (the system, of mass m) is: +mgh, by the normal force of the stairs. -mgh, by the normal force of the stairs. +mgh, by the gravitational force of the earth. -mgh, by the gravitational force of the earth 2. Relevant equations Gravitational Potential Energy = Mass*g*Height 3. The attempt at a solution I thought because the height was positive that the Work would therefore also be positive. And since we're gaining height that the system includes the earth, so I chose +mgh by the gravitational force of the earth.... This apparently isn't right. What am I doing wrong?