Calculating Work Done on a System

[KatlynEdwards]

Homework Statement

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

Homework Equations

Gravitational Potential Energy = Mass*g*Height

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?

 

[ahmadmz]

You have to consider which force is doing the work in addition to the direction. Gravity is the force that does work on you, and since it's direction is opposite to your motion, the work is does is negative.

 

[KatlynEdwards]

Ah I see what I did wrong. Thanks! :)