What is the Correct Moment of Inertia for a Unit Density Lamina?

Click For Summary
SUMMARY

The correct moment of inertia for a unit density lamina bounded by the curves \(y=\sqrt{4-x^2}\) and \(y=1-4x^2\) about the x-axis is calculated using the formula: \(2\left \{ \int_{0}^{2}\int_{0}^{\sqrt{4-x^2}}y^2 dy dx - \int_{0}^{1/2}\int_{0}^{1-4x^2}y^2 dy dx \right \}\). The error in the initial attempt arose from integrating the wrong limits, as the region of interest is only between the two curves, necessitating a split integral approach. A graphical representation of the region clarifies the bounds for integration.

PREREQUISITES
  • Understanding of moment of inertia concepts
  • Familiarity with double integrals in calculus
  • Knowledge of the curves \(y=\sqrt{4-x^2}\) and \(y=1-4x^2\)
  • Ability to sketch and interpret graphs of functions
NEXT STEPS
  • Study the derivation of moment of inertia for various shapes
  • Learn about the application of double integrals in physics
  • Explore graphical methods for visualizing regions of integration
  • Review the properties of parabolic curves and their intersections
USEFUL FOR

Students studying calculus, particularly those focusing on applications of double integrals and moment of inertia calculations in physics and engineering contexts.

longrob
Messages
51
Reaction score
0

Homework Statement


A lamina of unit density consists of the region between the two curves y=\sqrt{4-x^2} and y=1-4x^2 and the x axis.
Find it's moment of inertia about the x-axis.

Homework Equations


This is the correct answer:
2\left \{ \int_{0}^{2}\int_{0}^{\sqrt{4-x^2}}}y^2 dy dx - \int_{0}^{1/2}\int_{0}^{1-4x^2}y^2 dy dx \right \}
which I do understand.


The Attempt at a Solution


2\int_{0}^{2}\int_{1-4x^2}^{\sqrt{4-x^2}}y^2 dy dx
I don't understand why this is wrong.

Thanks in advance.
 
Physics news on Phys.org
Because you only want the moment of inertia BETWEEN the two curves. 1-4x^2 is negative for x>1/2. Sketch a graph of the region you want. You have to split the integral into two parts.
 
Thank you. That makes perfect sense !
 

Similar threads

Replies
4
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 2 ·
Replies
2
Views
2K
Replies
3
Views
2K
Replies
2
Views
1K
  • · Replies 2 ·
Replies
2
Views
2K
  • · Replies 6 ·
Replies
6
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 1 ·
Replies
1
Views
3K