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what is the direction of gravity, it's positive or negative?

user283144
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    What do you mean by "direction"? – nabla Nov 21 '15 at 00:00
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    You can use any coordinate system you want. The choice of coordinate system will determine the direction (and thus the sign) of any vector quantity. – HDE 226868 Nov 21 '15 at 00:02
  • Related: http://physics.stackexchange.com/q/11542/2451 and links therein. – Qmechanic Nov 21 '15 at 01:03
  • Sigh... voting to close – Alfred Centauri Nov 21 '15 at 02:01
  • The force of gravity, which is a vector quantity, therefore not positive or negative per se, in the sense that in a vector space $V$ one doesn't usually define an order and therefore if $\mathbf{v}\in V$ it may well make no sense to say that $\mathbf{v}$ is $>0$ or $<0$. What we can say is that the force of gravity exerced by a material point $P_1$ of mass $m_1$ on another point $P_2$ of mass $m_2$, whose distance is $r$, point from $P_2$ to $P_1$... – Self-teaching worker Nov 21 '15 at 11:43
  • ...i.e. it's $\frac{Gm_1 m_2}{r^2}\hat{\mathbf{r}}$ where $\hat{\mathbf{r}}$ is a unitary vector pointing from $P_2$ to $P_1$, which is the same as $-\frac{Gm_1 m_2}{r^2}\hat{\mathbf{u}}$ if $\hat{\mathbf{u}}$ is a unitary vector pointing from $P_1$ to $P_2$. – Self-teaching worker Nov 21 '15 at 11:44

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gravity, it's positive or negative?

The most convenient reference systems are the ones having the origin at a point on the surface of the earth or in the center of our planet, work with a negative acceleration due to gravity because g points to the origin.

By Symmetry
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pardaillan
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