Computing the polar moment of inertia (calculus)

Leo Liu
Messages
353
Reaction score
156
Homework Statement
.
Relevant Equations
.
Question:
1615904644504.png

Diagram:
1615904663519.png

So the common approach to this problem is using polar coordinates.
The definition of infinitesimal rotational inertia at O is ##dI_O=r^2\sigma\, dA##. Therefore the r. inertia of the triangle is
$$I_O=\int_{0}^{\pi/3}\int_{0}^{\sec\theta}r^2r\,drd\theta$$
whose value is ##=\sqrt 3 /2##.

But when I used rectangular coordinates to solve this problem, I got a different answer. The steps are shown below.
$$\xcancel{\begin{aligned}
I_O&=\int_{0}^{1}\int_{0}^{2x}x^2+y^2\,dydx\\
&=\int_0^1x^2y+\frac{y^3} 3\Bigg|_0^{2x}dx\\
&=\int_0^12x^3+\frac 8 3x^3\,dx\\
&=\frac{x^4} 2+\frac 2 3x^4\Bigg|_0^1=\frac 7 6
\end{aligned}}$$
Can someone please tell me where my mistakes are? Thanks!

Edit:
$$\begin{aligned}
I_O&=\int_{0}^{1}\int_{0}^{\sqrt 3 x}x^2+y^2\,dydx\\
&=\int_0^1x^2y+\frac{y^3} 3\Bigg|_0^{\sqrt 3 x}dx\\
&=\int_0^1 \sqrt 3 x^3+\sqrt 3 x^3\,dx\\
&=\frac{2\sqrt 3}{4}=\frac{\sqrt 3}2
\end{aligned}$$
 
Last edited:
Physics news on Phys.org
Perhaps the slope of the line is not 2 but a little less ... :wink:
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
Back
Top