You walk into an elevator, step on a scale, and push the "up" button. You recall that your normal weight is 625N. If the elevator has an acceleration of magnitude 2.50 m/s^2, what does the scale read? 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.