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- Thread starter relion65
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I think the coordinate system is arbitrary, so you could define gravitational acceleration to be positive or negative depending on how you set up your coordinate system.

If you mean the constant g, though, then that would be positive number (http://en.wikipedia.org/wiki/Standard_gravity). I think that's the convention.

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g = Gm(earth)/r2(earth) = 9.8ms/2

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Acceleration is a vector, not a number.So the final question: Here on earth, where gravity pulls u towards the center of the earth and hols u firmly down to the surface, is accerlation of gravity a negative or positive number?

If you look at a tiny portion of the surface of the Earth, this vector is nearly constant in magnitude and direction. You can treat this as constant without much loss of accuracy over this small area. Then you can define a local coordinate system in which one of the axes is directed along or against this local acceleration vector. If you choose this vertical axis (call it the z axis) to be positive upward then the acceleration due to gravity is [itex]-g\hat z[/itex]. If you choose the vertical axis to be positive downward then acceleration due to gravity is [itex]+g\hat z[/itex].

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Ken G

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