- #1
joyce1029
- 3
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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.
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.