# Finding the mass of a lamina given the density and the description of the lamina

• Mar 4th 2011, 01:50 AM
maximus101
Finding the mass of a lamina given the density and the description of the lamina
Let $W$ be the lamina in the plane enclosed by the curve $xy = 1$ and by the lines $y = (1/3) x$; $y = 5x$, i. e. ,
$W := [(x; y) |$ $(1/3) x$ $\le$ y $\le$ $5x$; $x$ $\ge$ $0$; $xy \le 1$ ].
Given that the density p(x; y) of the lamina is given by $p(x; y) = x^2 + y^2$ , calculate the mass of the
lamina.
the substitution u $= xy; v = y/x$could be used or otherwise.
• Mar 4th 2011, 04:58 AM
TKHunny
45 posts and not a hint of your own work?!

Here's a piece. You explain it to me and provide the other piece and the total evaluation.

$\int_{0}^{\frac{1}{\sqrt{5}}}\int_{\frac{x}{3}}^{5 \cdot x}\left(x^{2}+y^{2}\right)\;dydx$
• Mar 5th 2011, 04:20 AM
HallsofIvy
But that suggested substitution does make for an interesting simplification. The Jacobian gives $dxdy= vdudv$ so the integrand becomes $(u+ uv^2)dudv$.
• Mar 5th 2011, 09:45 AM
maximus101
Quote:

Originally Posted by TKHunny
45 posts and not a hint of your own work?!

Here's a piece. You explain it to me and provide the other piece and the total evaluation.

$\int_{0}^{\frac{1}{\sqrt{5}}}\int_{\frac{x}{3}}^{5 \cdot x}\left(x^{2}+y^{2}\right)\;dydx$

I think that integral you gave will give the density of the lamina, and we then have to find the surface area of it, then $MASS= SURFACE AREA * DENSITY$

however I'm not sure how you came to this integral and chose those limits, and do I have to apply $dy$ to the inside integral and then $dx$ to the result of the inside?

I drew the lamina and I think the upper and lower limit you chose for the inside uses the space between the straight lines and then on the outside integral I do not know how you chose $0 and 1/\sqrt{5}$ but I know that the line $y=5x$ meet the curve at $(\sqrt{5}/5 , \sqrt{5})$
• Mar 5th 2011, 09:49 AM
maximus101
Quote:

Originally Posted by HallsofIvy
But that suggested substitution does make for an interesting simplification. The Jacobian gives $dxdy= vdudv$ so the integrand becomes $(u+ uv^2)dudv$.

once I make that substitution and the integrand becomes $(u+ uv^2)dudv$

how do I solve the integral and also do I have to change the limits,

I think I have to integrate the inside one first w.r.t du then the outer one w.r.t dv?
limits of the inside will change using $y=u/x$ and the outer one will use $x=y/v$