- #1
dingo_d
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Homework Statement
Distribution of matter is given in cylindrical coordinates:
[tex]\rho(\vec{r})=\frac{1}{\rho}\delta(\rho^2-10\rho+9)\delta\left(\frac{z^2-a^2}{z^2+a^2}\right)\delta(\cot(\phi))[/tex]
where [tex]a>0[/tex] is a constant. Find the complete mass of the object.
Homework Equations
The mass of the object is given as:
[tex]M=\int_{\partial V}\rho(\vec{r})dV[/tex]
So in order to calculate the integral I need to expand the deltas with this formula:
[tex]\delta(f(x))=\sum_{i=1}^n\frac{\delta(x-x_i)}{|f'(x_i)|}[/tex], where [tex]|f'(x_i)|[/tex] is the derivative of the function evaluated at the zeroes of the function [tex]x_i[/tex].
Integral in cylindrical coordinate system is:
[tex]\int_0^\infty\rho d\rho\int_0^{2\pi} d\phi\int_{-\infty}^\infty dz[/tex]
The Attempt at a Solution
I have expanded the first delta:
[tex]\delta(\rho^2-10\rho+9)=\frac{\delta(\rho-1)}{8}+\frac{\delta(\rho-9)}{8}[/tex].
I'm having problem with the second one. Do I 'brake' it with partial fractions?
Third one (which I'm not sure I got it right):
[tex]\delta(\cot(\phi))=\sum_{m=-\infty}^\infty\delta\left(\phi-\left(m+\frac{1}{2}\right)\pi\right)[/tex]
So my problem is: how do I solve the second delta and I'm not quite sure I got the third one right. How should I integrate it?