Oiy. It is my 6th year teaching physics and early on, I diverged from the textbook (Holt Modern Physics) regarding how they handled "g". To me, it made more sense that "g" was -9.81m/s2 , while the text handled it as "g" = 9.81 m/s2, and negatives are assigned directionally. Overall, my approach (I felt) simplified calculations. When we do sum of forces I handled it as actually adding forces on each axis and assigning + or - to the force as indicated by cartesian direction (since "g" is negative, this automatically makes Fg a negative number. Life is good! If on the y axis a=zero then ΣFy= Fg + Fn=0, therefore Fn is positive. Life is still good! When things begin to move on the x and we often default to the acceleration being in the positive direction, then Fk, is a negative. OK, seems reasonable. Then we get to μ! μk= Fk/Fn. When using this to create an expression for Fn, this almost always renders a negative normal force! This makes it difficult to just trust the signs and be careful with the algebra. Sigh. I muddle through it with some fudging every year, but I want to do better than that. Help! Should I just go back and train myself to handle it the way the text does? Is there some brilliant 3rd road that I am missing? Thanks in advance.