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/s(adsbygoogle = window.adsbygoogle || []).push({}); ^{2}, while the text handled it as "g" = 9.81 m/s^{2}, 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 F_{g}a negative number. Life is good!

If on the y axis a=zero then ΣF_{y}= F_{g}+ F_{n}=0, therefore F_{n}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 F_{k}, is a negative. OK, seems reasonable.

Then we get to μ!

μ_{k}= F_{k}/F_{n}.

When using this to create an expression for F_{n}, 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.

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# Sign dilemma with coefficients of friction

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