- #1
physicsod
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Hello! I'm trying to derive the formula for the moment of inertia of a solid sphere, and I keep running into a strange solution.
I set up the infinitesimally mass of an infinitesimally thin "shell" of the sphere:
dm = 4[itex]\rho[/itex][itex]\pi[/itex]r2 dr
And then solved for the moment of inertia:
I = [itex]\int[/itex]r2dm
= [itex]\int[/itex]r2(4[itex]\rho[/itex][itex]\pi[/itex]r2 dr)
= 4[itex]\rho[/itex][itex]\pi[/itex][itex]\int[/itex]r4 dr
= (4/5)[itex]\rho[/itex][itex]\pi[/itex]r5
And solving for [itex]\rho[/itex] we get the following:
[itex]\rho[/itex] = M/((4/3)[itex]\pi[/itex]r3).
Substituting that into the previously solved equation for I, I get the following:
I = (3/5)Mr3.
What am I doing wrong? I know the formula involves a coefficient of 2/5, not 3/5, but I can't find my problem.
Thank you in advance!
I set up the infinitesimally mass of an infinitesimally thin "shell" of the sphere:
dm = 4[itex]\rho[/itex][itex]\pi[/itex]r2 dr
And then solved for the moment of inertia:
I = [itex]\int[/itex]r2dm
= [itex]\int[/itex]r2(4[itex]\rho[/itex][itex]\pi[/itex]r2 dr)
= 4[itex]\rho[/itex][itex]\pi[/itex][itex]\int[/itex]r4 dr
= (4/5)[itex]\rho[/itex][itex]\pi[/itex]r5
And solving for [itex]\rho[/itex] we get the following:
[itex]\rho[/itex] = M/((4/3)[itex]\pi[/itex]r3).
Substituting that into the previously solved equation for I, I get the following:
I = (3/5)Mr3.
What am I doing wrong? I know the formula involves a coefficient of 2/5, not 3/5, but I can't find my problem.
Thank you in advance!