Using Newtonian field equation to find the gravity inside a sphere

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SUMMARY

The discussion focuses on calculating the gravitational field at a distance of r=R/2 from the center of a spherically symmetric body with radius R using Newtonian field equations. Key equations include the Poisson equation, \(\nabla^{2}\psi=4\pi G \rho\), and the gravitational potential \(\psi=G\int_{V'}\rho\frac{1}{|\vec{r}-\vec{r^{'}}|}d^{3}\vec{r^{'}}\). The volume calculations for the inner and outer spheres are critical, with the final result yielding \(\frac{7}{6}\rho \pi r^{3}\). An important correction noted in the discussion is the inclusion of a G*2π factor in one of the integrals.

PREREQUISITES
  • Understanding of Newtonian gravity and gravitational fields
  • Familiarity with the Poisson equation in physics
  • Knowledge of volume integrals in spherical coordinates
  • Basic calculus, particularly integration techniques
NEXT STEPS
  • Study the derivation of the Poisson equation in gravitational contexts
  • Learn about gravitational potential and field calculations in spherical symmetry
  • Explore advanced integration techniques in multiple dimensions
  • Investigate the implications of adding constants in gravitational equations
USEFUL FOR

Students and professionals in physics, particularly those focusing on gravitational theory, astrophysics, and applied mathematics. This discussion is beneficial for anyone looking to deepen their understanding of gravitational fields in spherical bodies.

lostminty
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Homework Statement



work out g field at a distance r=R/2 from the centre of a spherically symmetric body of radius R.

Homework Equations



[itex]\nabla^{2}\psi=4\pi G \rho[/itex]

[itex]\psi=G\int_{V'}\rho\frac{1}{|\vec{r}-\vec{r^{'}}|}d^{3}\vec{r^{'}}[/itex]

[itex]-\int_{V}\nabla\cdot g dV = \int_{V}\rho dV[/itex]

V=4/3∏r3

The Attempt at a Solution



V=4/3∏r3

V1=4/3∏(R/2)3 = 1/6∏R3
V2=4/3∏R3
[itex]\int_{1/6\pi R^{3}}^{4/3\pi R^{3}}\rho dV[/itex]

[itex]=\rho(4/3\pi R^{3}-1/6\pi R^{3})[/itex]

[itex]=\frac{7}{6}\rho \pi r^{3}[/itex]
 
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lostminty said:

Homework Statement



work out g field at a distance r=R/2 from the centre of a spherically symmetric body of radius R.

Homework Equations



[itex]\nabla^{2}\psi=4\pi G \rho[/itex]

[itex]\psi=G\int_{V'}\rho\frac{1}{|\vec{r}-\vec{r^{'}}|}d^{3}\vec{r^{'}}[/itex]

[itex]-\int_{V}\nabla\cdot g dV = \int_{V}\rho dV[/itex]

V=4/3∏r3

The Attempt at a Solution



V=4/3∏r3

V1=4/3∏(R/2)3 = 1/6∏R3
V2=4/3∏R3
[itex]\int_{1/6\pi R^{3}}^{4/3\pi R^{3}}\rho dV[/itex]

[itex]=\rho(4/3\pi R^{3}-1/6\pi R^{3})[/itex]

[itex]=\frac{7}{6}\rho \pi r^{3}[/itex]


should be a G*2*pi in the front of one of those integrals
 

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