For a mass on an incline plane, is the downward force mg cos Θ or -mg cos Θ? I'm inclined to state there should be a negative but from my lectures, the negative does not appear to be stipulated. I was wondering if it's was a mistake by the lecturer. mg cos Θ + FN = 0 mg cos Θ = -FN the downward force must be of equal magnitude to the normal force-but opposite direction. This makes sense mathematically. However, geometrically, it does make sense for the down ward force to be -mg cos Θ. Could someone clarify?