# Thread: First Moment about the Line x=4

1. ## First Moment about the Line x=4

I did the following question two ways; with vertical strips and with horizontal strips. I got two slightly different answers [ rho*(13/4 + 6ln(2)) with vertical strips and rho*(13/4 - 6ln(2)) with horizontal strips]. I can't figure out which (if either) is the correct answer or where I went wrong.

Here is the question:

Consider the thin plate of constant density rho [mass/unit area] bounded by the curves:
y=ln(x), y=0, and x=2
Find the first moment (moment of mass) of this plate about the line x=4.

2. Hi

Are you talking about the moment of inertia

$\int_{1}^{2} \int_{0}^{\ln x} \rho (4-x)^2 dy dx$ ?

3. No. I'm talking about moment of mass (first moment). Also, we are just using single integrals at the moment.