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I know how to solve this problem, but I don't know why I'm getting the wrong answer.

I first defined up as positive because it is the direction of acceleration. Then there are two forces: the normal and gravity.

m = w/g

m = (625/9.8) = 63.8 kg

N - mg = ma

N = ma + mg

N = m(a+g)

N = 63.8(2.5+(-9.8)) = -465.74N

Which is incorrect. This is correct if I make gravity positive, but I don't understand why it should be positive if I defined up as positive and gravity points down.