How Do You Calculate the Moment of Inertia of a Hollow Sphere?

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SUMMARY

The moment of inertia of a hollow sphere can be calculated using the formula I = (2/3) * m * r^2, where m is the mass and r is the radius of the sphere. The discussion highlights the importance of distinguishing between the distance from the axis of rotation and the center of the sphere when performing the integration. The correct approach involves using surface density (σ) and the differential area (dA) to compute the moment of inertia accurately. The initial miscalculation arose from incorrect integration limits and variable definitions.

PREREQUISITES
  • Understanding of integral calculus and its applications in physics.
  • Familiarity with the concepts of moment of inertia and rotational dynamics.
  • Knowledge of surface density and its relation to mass and area.
  • Basic principles of spherical geometry and volume calculations.
NEXT STEPS
  • Study the derivation of the moment of inertia for different geometric shapes, including solid and hollow cylinders.
  • Learn about the application of the parallel axis theorem in calculating moments of inertia.
  • Explore advanced integration techniques used in physics, particularly in three-dimensional space.
  • Investigate the relationship between mass distribution and rotational motion in rigid bodies.
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Students in physics, mechanical engineers, and anyone studying dynamics and rotational motion who require a clear understanding of calculating moments of inertia for various shapes.

christikiki22
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Homework Statement



Calculate the moment of inertia of a spherical shell (i.e. hollow sphere) of uniform surface density about an axis passing through its center.

Homework Equations


The Attempt at a Solution



Integral ( r^2 * dm)
Integral (r^2 * p * dV) ... where p=density
p * Integral (r^2 * dV)

where V= (4/3)* pi * r^3
and dV= 4 * pi* r^2
therefore,

p * Integral (r^2 *4 * pi* r^2)
4*p*pi* Integral (r^4)
4*p*pi* ((r^5)/5) from 0 to r

p=density=m/V= m/(4/3 * pi * r^3)

4*m/(4/3 * pi * r^3)*pi* ((r^5)/5

(3*m*r^2)/5

but it should be (2*m*r^2)/3.
Please help!
 
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christikiki22 said:
Integral ( r^2 * dm)
...
and dV= 4 * pi* r^2
The r in the first expression represents the distance from the axis of rotation to the point in question while the r in the second expression represents the distance from the center of the sphere to the point in question. Those are two different things.
 
A hollow sphere is a two-dimensional surface, so you need to use dm = \sigma dA, where \sigma is the surface density.
 

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