Derivation of moment of inertia of a solid sphere

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SUMMARY

The derivation of the moment of inertia for a solid sphere, represented as I = 2/5 mR^2, can be approached using the method of shells. The user initially attempted to derive this using the integral of r^2dm, but encountered an error resulting in I = 3/5 mR^2. The key misunderstanding lies in the assumption that all mass in a thin shell is at the same distance from the axis of rotation, which is not the case for spherical shells. Correctly applying the concept of moment of inertia for a thin shell is crucial for accurate derivation.

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  • Understanding of moment of inertia concepts
  • Familiarity with integral calculus
  • Knowledge of spherical coordinates
  • Basic principles of mass distribution in physics
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Students studying physics, particularly those focusing on mechanics and rotational dynamics, as well as educators looking for clarification on the derivation of moment of inertia for solid objects.

joe5185
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Homework Statement


I need to know why my derivation does not work. I am attempting to derive I=2/5 mR^2

Homework Equations


I have seen people derive it using disks but my question is why do the shells not work? Where in my set up did I go wrong? Thanks

The Attempt at a Solution


I am attempting to use shells to do this.
integral(r^2dm)=I
Ro=p=m/v so pdv=dm
dv=4(pi)r^2dr
moment of inertia (I) =definite integral from r=0 to r=R (4p(pi)r^4dr)

When I solve this integral it becomes (3/5)mR^2 and not (2/5) mR^2
 
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Hello, and welcome to PF!

joe5185 said:
I am attempting to use shells to do this.
integral(r^2dm)=I
What is the moment of inertia of a single thin shell of mass dm and radius r? (Hint: It is not r2dm)
 
TSny said:
Hello, and welcome to PF!What is the moment of inertia of a single thin shell of mass dm and radius r? (Hint: It is not r2dm)
Well in general moment of inertia R^2dm and I am using the thin shells to eventually make up the entire volume so I can sub out for dm in terms of r
 
If all the mass in the element dm is at the same distance r from the axis of rotation, then the moment of inertia about that axis will be r2 dm. For a spherical shell, is all of the mass in the shell at the same distance from the axis?
 
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oh I see. thank you
 

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