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**Coordinate System--Spring Vertical**

Hi! This is a question on the use of a coordinate system.

In the princeton review, I don't understand the coordinate system they are using it; it doesn't make sense. That is, for a vertical spring, the net force on the mass is kx-mg. But, shouldn't it be mg-kx? The component of spring vertical is -kx, not +kx. What type of coordinate system would have +kx? This may be a trivial matter, but any help to ease my slight confusion would help! Thanks.