# Why is this wrong? (gravitational flux)

## Main Question or Discussion Point

in my notes i have..

$\Phi$ = $\int$ $\vec{g}\cdot$dA = g(4$\pi$r2) = -$\frac{GM}{r^2}$(4$\pi$r2)

which yields $\vec{g}$=-$\frac{GM}{r^2}$

here's what i did independently..

$\Phi$ = $\int$ $\vec{g}\cdot$dA = -$\frac{GM}{r^2}$(4$\pi$r2)

but since $\vec{g}$= -$\vec{\nabla}$$\Phi$

ie...

$\vec{g}$=$\frac{d \Phi}{dr}$=$\frac{4 \pi GM}{r}$

but this comes out to a different answer. is this still correct?

*to admins/moderators/mentors/etc: before i get in trouble again, i appologize if this is considered another hw problem. but i thought it wasn't, so i posted in this section

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