Short and Sweet Calculus

7.3 Volumes of Solids With Known Cross Sections: The Slice Method

The volumes of many solids can be obtained by the application of the slice method. In the previous section, we sliced the solid into infinitely many thin disks. However, the element of volume does not have to be necessarily a disk (or a washer).

In a more general case, imagine the solid is sliced into infinitesimally thin slices of thickness \(dx\) by a family of planes perpendicular to the \(x\)-axis (Figure 7.10). Suppose you know a formula for the area of an arbitrary cross section of the solid made by such planes \(A(x)\). Some common cross sections are triangles, squares, rectangles, trapezoids, and semicircles. Then the volume of a slice (the element of volume) is this area \(A(x)\) multiplied by the thickness of the slide \(dx\): \[dV=A(x)dx.\] The total volume of the solid is the sum of the volumes of these slices. If the solid is bounded by two parallel planes perpendicular to the \(x\)-axis at \(x=a\) and \(x=b\), then \[\boxed{V=\int_{a}^{b}dV=\int_{a}^{b}A(x)dx.}\]

Similarly, suppose that the solid is cut by planes perpendicular to the \(y\)-axis and \(A(y)\) is the area of an arbitrary cross section (Figure 7.11). Then \[\boxed{V=\int_{c}^{d}A(y)dy}\] if the solid is bounded between the planes \(y=c\) and \(y=d\) (\(c<d\)).

Example 7.5. Consider a right circular cylinder of radius \(a\). Find the volume of the wedge cut from this cylinder by a plane through a diameter of the base of the cylinder making a \(45^{\circ}\) angle with the base.

Solution

Method (a): Let’s cut the wedge into thin triangular slices of cross section area \(A(x)\) with planes perpendicular to the edge of the wedge as shown in Figure 7.12.

The base of the triangle is \(b=\sqrt{a^{2}-x^{2}}\) and its height is \(h=b\tan45^{\circ}=\sqrt{a^{2}-x^{2}}\). Thus \[A(x)=\frac{1}{2}bh=\frac{1}{2}\left(a^{2}-x^{2}\right)\] and the volume of the infinitely thin slice is \[dV=A(x)dx=\frac{1}{2}\left(a^{2}-x^{2}\right)dx\] Because \(x\) varies between \(-a\) and \(a\) the total volume is

\[\int_{-a}^{a}dV=\int_{-a}^{a}\frac{1}{2}\left(a^{2}-x^{2}\right)dx=2\int_{0}^{a}\frac{1}{2}\left(a^{2}-x^{2}\right)dx=\left[a^{2}x-\frac{1}{3}x^{3}\right]_{0}^{a}=\frac{2a^{3}}{3}.\] Method (b): We can cut the wedge into thin slices of rectangular cross section area \(A(y)\) with planes parallel to the wedge as shown in Figure 7.13.

The width of the rectangle is \(2\sqrt{a^{2}-y^{2}}\) and the height is \(y\). Therefore, the area of the rectangle is \[A(y)=2y\sqrt{a^{2}-y^{2}},\] and the volume of the slice is \[dV=A(y)dy=2y\sqrt{a^{2}-y^{2}}\] Because \(y\) varies between \(0\) and \(a\), the total volume of the wedge is: \[V=\int_{0}^{a}dV=\int_{0}^{a}2y\sqrt{a^{2}-y^{2}}dy\] To evaluate the above integral, let \(u=a^{2}-y^{2}\). Then \[du=-2ydy\] \[y=0\Leftrightarrow u=a^{2}\] \[y=a\Leftrightarrow u=0\] and \[V=\int_{0}^{a}2y\sqrt{a^{2}-y^{2}}\,dy=\int_{0}^{a}\underbrace{\sqrt{a^{2}-y^{2}}}_{\sqrt{u}}\underbrace{2y\,dy}_{-du}=-\int_{a^{2}}^{0}\sqrt{u}\,du=\int_{0}^{a^{2}}\sqrt{u}\,du=\frac{2}{3}\left.u^{\frac{3}{2}}\right|_{0}^{a^{2}}=\frac{2}{3}a^{3},\] as before.


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