Shape of Minimum Moment of Inertia for Fixed Volume: How to Prove?

Click For Summary
SUMMARY

The discussion centers on determining the shape of a solid of revolution that minimizes the moment of inertia while maintaining a fixed volume. The relevant equations include the moment of inertia formula, I = ∫(1/2)r²dm, and the volume equation, V = ∫πr²dz. The conclusion drawn is that the optimal shape is a cylinder, despite initial assumptions that it might be a sphere. The application of the Euler-Lagrange equations is essential for proving this result.

PREREQUISITES
  • Understanding of calculus, specifically integral calculus.
  • Familiarity with the concepts of moment of inertia and solid of revolution.
  • Knowledge of the Euler-Lagrange equations in variational calculus.
  • Basic principles of physics related to rotational dynamics.
NEXT STEPS
  • Study the derivation of the Euler-Lagrange equations in variational calculus.
  • Explore the properties of moment of inertia for various geometric shapes.
  • Investigate the implications of fixed volume on the shape of solids in physics.
  • Learn about optimization techniques in calculus, particularly in the context of physical systems.
USEFUL FOR

Students and professionals in mechanical engineering, physics, and applied mathematics who are interested in optimization problems related to solid shapes and their physical properties.

Grand
Messages
74
Reaction score
0

Homework Statement


Find the shape of the solid of revolution with the minimum moment of inertia about it's axis, given the volume is fixed.

Homework Equations


[tex]I=\int\frac{1}{2}r^2dm=\int\frac{1}{2}r^2\rho~dV=\int\frac{\rho\pi}{2}r^4dz[/tex]
[tex]V=\int \pir^2dz[/tex]

[tex]I+\lambda V[/tex] must be stationary.

The Attempt at a Solution


However, when I apply the E-L equs nothing meaningful comes out.
 
Physics news on Phys.org
it must be a sphere.
 
How do you prove this. The answer is cylinder.
 

Similar threads

Replies
17
Views
2K
  • · Replies 17 ·
Replies
17
Views
2K
  • · Replies 3 ·
Replies
3
Views
7K
  • · Replies 4 ·
Replies
4
Views
4K
  • · Replies 7 ·
Replies
7
Views
6K
  • · Replies 9 ·
Replies
9
Views
2K
  • · Replies 4 ·
Replies
4
Views
2K
  • · Replies 2 ·
Replies
2
Views
5K
  • · Replies 3 ·
Replies
3
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K