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## Homework Statement

A monument is made from stone blocks of density 3800kg/m^3. The monument is 15.7m high, 64.8m wide at the base, and 3.6m thick from front th back. How much work was required to build the monument? (Hint: find ycm).

## Homework Equations

ycm = (1/M) * ∫ydm, M = mass total

## The Attempt at a Solution

Take a cross section and get a rectangle.

The length will be: (64.8/15.7) * y

The width will be: (3.6/15.7) * y

Thus, dm = density * length * width * dy

M total = density * volume = 6958742.8

ycm = (1/M) * ∫y * density * length * width * dy (from 0 to 15.7)

Okay, so this solution is wrong. It gives the the ycm as 7.85m. But ycm is actually a third of the height (as it is for triangles).

My question is: what is fundamentally wrong with my approach?

I have the solution though. I don't need that.