# Find the centroid of the solid region ?

by CalleighMay
Tags: centroid, region, solid
 HW Helper P: 2,616 Check out the double integral formulae for center of mass. It goes like this: $$\bar{x} = \frac{M_y}{M}$$ $$\bar{y} = \frac{M_x}{M}$$ where $$M = \iint_R \delta (x,y) dA$$. where $$\bar{x}, \bar{y}$$ refers to the coordinates of the centre of mass.
 HW Helper P: 1,662 I'm assuming that your calculus sequence is much like that where I work, so your Calculus I and II only deal with single-variable calculus. (I surmise this since your problem is supposed to be a "preview" of something in Calculus III.) Multiple integration is the "big hammer" by which this problem can be easily beaten down. But when you don't have a particular tool available, you sometimes have to be clever instead. Have you drawn a picture of this figure? I won't say just now what it looks like, but you should notice that it is symmetric about the y-axis. So you already know one coordinate of the centroid! It remains to find the other two coordinates, using only single-variable integration. So we'll have to recast this problem accordingly. Look at the infinitesimal "slices" of this figure parallel to the yz-plane. What shape are they? (They will also all be similar, in the geometical sense of the word.) What do you know about the centroid of that shape? (If you haven't worked it out already, it will take only a minute to do so...) We are going to integrate "slices" along the x-axis. Because of the figure's symmetry, we only need to integrate from x = 0 to x = (what?). [The other half makes a mirror-image contribution, which is unnecessary to evaluate for finding the centroid.] The "slices" have to be "weighted" with an infinitesimal mass dm, which is given by the density times the area of each slice (as a function of x) times dx. So the coordinates of the centroid will be $$x_C = 0$$ $$y_C = \frac{\int_{0}^{a} \rho \cdot y(x) \cdot A(x) \ dx }{M}$$ and $$z_C = \frac{\int_{0}^{a} \rho \cdot z(x) \cdot A(x) \ dx }{M}$$ with $$M = \int_{0}^{a} \rho \cdot A(x) \ dx$$ . That's as much as I'm saying for now. You should find the upper limit a and work out the details before we can discuss this further on the Forum... EDIT: I thought of an even more direct way to do this. Looking at the figure, think about the curve that the centroids of the "vertical slices" (parallel to the yz-plane) would sweep out in space. What shape is that? How do you find the centroid of such a curve? How would it be positioned in three dimensions? (Again, the problem can be worked out with single-variable calculus, and you may have already found the centroid of this curve in your earlier examples in lectures or the book or in homework problems...)