# Thread: Moments, finding coordinates of the centroid of an area

1. ## Moments, finding coordinates of the centroid of an area

I don't know what I'm doing wrong; I'm doing everything the book tells me I need to do-and i've looked at their examples but I keep getting an answer similar but not quite what is in the back of the book. Here are the answers in the back for this problem: (-4/5, -32/7) and I got (-8/5, -96/7)

Anyone see what I'm missing here? Or can anyone show me how you would approach this problem (with only the given curves for information)

Find the coordinates of the centroid of the area bounded by the given curves y=x³ , y=-8 , x=0

M= ∫(upper limit 0 and lower limit -2) x³ dx = ¼x^4 (from 0 to -2) = -4

My= ∫(ul 0 and ll -2) x·x³ dx = 1/5 x^5 (from 0 to -2) = 0-(-32/5) = 32/5

Mx= ∫(ul 0 and ll -8) y·[y^(1/3)] dy= 3/7 [y^(7/3)] (from 0 to -8) = 0-(384/7)= 384/7

M·y̅= Mx, so

-4·y̅= 384/7, y̅= -96/7

M·x̅=My, so

-4·x̅= 32/5, x̅=-8/5

2. ## Re: Moments, finding coordinates of the centroid of an area

Originally Posted by CoffeeBird
I don't know what I'm doing wrong; I'm doing everything the book tells me I need to do-and i've looked at their examples but I keep getting an answer similar but not quite what is in the back of the book. Here are the answers in the back for this problem: (-4/5, -32/7) and I got (-8/5, -96/7)

Anyone see what I'm missing here?

Find the coordinates of the centroid of the area bounded by the given curves y=x³ , y=-8 , x=0

M= ∫(upper limit 0 and lower limit -2) x³ dx = ¼x^4 (from 0 to -2) = -4
This is NOT the region between y= -8 and $\displaystyle y= x^3$, it is the region between y= 0 and $\displaystyle y= x^3$.

My= ∫(ul 0 and ll -2) x·x³ dx = 1/5 x^5 (from 0 to -2) = 0-(-32/5) = 32/5

Mx= ∫(ul 0 and ll -8) y·[y^(1/3)] dy= 3/7 [y^(7/3)] (from 0 to -8) = 0-(384/7)= 384/7

M·y̅= Mx, so

-4·y̅= 384/7, y̅= -96/7

M·x̅=My, so

-4·x̅= 32/5, x̅=-8/5

3. ## Re: Moments, finding coordinates of the centroid of an area

you mean the limits of integration are incorrectly chosen? can you help me figure out what they should be?

4. ## Re: Moments, finding coordinates of the centroid of an area

ok i see what happened.