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Z-coordinate of centroid , centre of mass prob

Hi!

**Problem:** Determine the z-coordinate of the centroid of the solid generated by revolving the quarter-circular area thorugh 90 degress about the z-axis.

**Solution attempt:**

I tried to calculate the volume of the solid. I got it to be

I have some trouble setting up the sum of moments integral, that is

I tried to use a horizontal strip as differential element:

Witdh:

Height:

Moment arm:

But I also need thickness, right? expressed in terms of z

Thx!