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pleasehelp12
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gravity inside a solid sphere
I'm having a hard time setting up a triple integral to find the force of gravity inside a solid sphere. I've done a similar proof in physics before with gravity inside a spherical shell, but it only required a single integral. In this problem the answer must be derived using a triple integral.
Newton's law of gravity:
|[tex]\vec{F_{grav}}[/tex]|= [tex]\frac{GmpdV}{r^{2}}[/tex]
I believe i found a way to set up the triple integral using spherical coordinates on another physics forum thread, but I don't understand how to get the integrand. Can someone please explain the way the intergrand was derived in the following integral?
Homework Statement
I'm having a hard time setting up a triple integral to find the force of gravity inside a solid sphere. I've done a similar proof in physics before with gravity inside a spherical shell, but it only required a single integral. In this problem the answer must be derived using a triple integral.
Homework Equations
Newton's law of gravity:
|[tex]\vec{F_{grav}}[/tex]|= [tex]\frac{GmpdV}{r^{2}}[/tex]
The Attempt at a Solution
I believe i found a way to set up the triple integral using spherical coordinates on another physics forum thread, but I don't understand how to get the integrand. Can someone please explain the way the intergrand was derived in the following integral?
arildno said:And, if you want to prove it mathematically using Newton's general law of gravitation and calculus techniques, here is most of it:
1. In spherical polar coordinates, let the position of a mass particle inside the ball be given by [itex](\hat{r},\theta,\phi)[/itex] (measured from the C.M)
where [itex]\phi[/itex] is the angle between the vertical and the particle's position vector.
Let the density be constant for all sphere particles and the radius of the sphere R.
2. Consider a test particle P having mass m and position vector [itex]r\vec{k}[/itex], i.e, a distance r along the "vertical"
3. We need to sum up all forces acting on P from sphere particles, i.e, compute the integral:
[tex]\vec{F}=-G\rho{m}\int_{0}^{R}\int_{0}^{\pi}\int_{0}^{2\pi}\frac{\hat{r}^{2}\sin\phi((r-\hat{r}\cos\phi)\vec{k}-\hat{r}(\sin\phi(\cos\theta\vec{i}+\sin\theta\vec{j}))}{(\hat{r}^{2}+r^{2}-2r\hat{r}\cos\phi)^{\frac{3}{2}}}d\theta{d\phi}d\hat{r}[/tex]
where G is the universal gravitation constant and [itex]\rho[/itex] is the density of sphere particles.
4. It is easy to see that the horizontal plane components vanishes; the [itex]\phi[/itex]-integration is then best handled by integration by parts.
In the [itex]\hat{r}[/itex] integration, take care of whether you have [itex]r<\hat{r}[/itex] or [itex]r>\hat{r}[/itex]
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