Learning Objectives
- Recognize the main features of ellipsoids, paraboloids, and hyperboloids.
- Use traces to draw the intersections of quadric surfaces with the coordinate planes.
Quadric Surfaces
We have learned about surfaces in three dimensions described by first-order equations; these are planes. Some other common types of surfaces can be described by second-order equations. We can view these surfaces as three-dimensional extensions of the conic sections we discussed earlier: the ellipse, the parabola, and the hyperbola. We call these graphs quadric surfaces.
Quadric surfaces are the graphs of equations that can be expressed in the form
[latex]Ax^2+By^2+Cz^2+Dxy+Exz+Fyz+Gx+Hy+Jz+K=0[/latex].
When a quadric surface intersects a coordinate plane, the trace is a conic section. Since these conic sections show up frequently in this section, we review some commonly encountered equations briefly below:
Recall: Equations of Conic Sections
The equation [latex] x^2 = 4py [/latex], where [latex] p [/latex] is a constant, represents a parabola; the parabola is opening upward if [latex] p>0 [/latex] and downward if [latex] p<0 [/latex]. Similarly, the equation [latex] y^2 = 4px [/latex], where [latex] p [/latex] is a constant, also represents a parabola; the parabola is opening rightward if [latex] p>0 [/latex] and leftward if [latex] p<0 [/latex].
The equation [latex] \frac{x^2}{a^2} + \frac{y^2}{b^2} = 1 [/latex], where [latex] a \text{ and } b [/latex] are constants, represents an ellipse. It has a horizontal major axis if [latex] a > b > 0 [/latex] and a vertical major axis if [latex] b > a > 0 [/latex]. The length of its horizontal axis is [latex] 2a [/latex] and the length of its vertical axis is [latex] 2b [/latex].
The equation [latex] \frac{x^2}{a^2} - \frac{y^2}{b^2} = 1 [/latex], OR the equation [latex] \frac{y^2}{b^2} - \frac{x^2}{a^2} = 1 [/latex] where [latex] a \text{ and } b [/latex] are constants, represents an hyperbola. The hyperbola has two branches opening either to the right and left, in the case of the first equation, or up and down, in the case of the second equation.
An ellipsoid is a surface described by an equation of the form [latex]\frac{x^2}{a^2}+\frac{y^2}{b^2}+\frac{z^2}{c^2}=1[/latex]. Set [latex]x=0[/latex] to see the trace of the ellipsoid in the [latex]yz[/latex]-plane. To see the traces in the [latex]xy[/latex]– and [latex]xz[/latex]-planes, set [latex]z=0[/latex] and [latex]y=0[/latex], respectively. Notice that, if [latex]a=b[/latex], the trace in the [latex]xy[/latex]-plane is a circle. Similarly, if [latex]a=c[/latex], the trace in the [latex]xz[/latex]-plane is a circle and, if [latex]b=c[/latex] then the trace in the [latex]yz[/latex]-plane is a circle. A sphere, then, is an ellipsoid with [latex]a=b=c[/latex].
Example: sketching an ellipsoid
Sketch the ellipsoid [latex]\frac{x^2}{2^2}+\frac{y^2}{3^2}+\frac{z^2}{5^2}=1[/latex].
Show Solution
Start by sketching the traces. To find the trace in the [latex]xy[/latex]-plane, set [latex]z=0[/latex]: [latex]\frac{x^2}{2^2}+\frac{y^2}{3^2}=1[/latex] (see Figure 1). To find the other traces, first set [latex]y=0[/latex] and then set [latex]x=0[/latex].
Figure 1. (a) This graph represents the trace of equation [latex]\frac{x^2}{2^2}+\frac{y^2}{3^2}=1[/latex] in the [latex]xy[/latex]-plane, when we set [latex]z=0[/latex]. (b) When we set [latex]y=0[/latex], we get the trace of the ellipsoid in the [latex]xz[/latex]-plane, which is an ellipse. (c) When we set [latex]x=0[/latex], we get the trace of the ellipsoid in the [latex]yz[/latex]-plane, which is also an ellipse.
Now that we know what traces of this solid look like, we can sketch the surface in three dimensions (Figure 2).
Figure 2. (a) The traces provide a framework for the surface. (b) The center of this ellipsoid is the origin.
The trace of an ellipsoid is an ellipse in each of the coordinate planes. However, this does not have to be the case for all quadric surfaces. Many quadric surfaces have traces that are different kinds of conic sections, and this is usually indicated by the name of the surface. For example, if a surface can be described by an equation of the form [latex]\frac{x^2}{a^2}+\frac{y^2}{b^2}=\frac{z}{c}[/latex], then we call that surface an elliptic paraboloid. The trace in the [latex]xy[/latex]-plane is an ellipse, but the traces in the [latex]xz[/latex]-plane and [latex]yz[/latex]-plane are parabolas (Figure 3). Other elliptic paraboloids can have other orientations simply by interchanging the variables to give us a different variable in the linear term of the equation [latex]\frac{x^2}{a^2}+\frac{z^2}{c^2}=\frac{y}{b}[/latex] or [latex]\frac{y^2}{b^2}+\frac{z^2}{c^2}=\frac{x}{a}[/latex].
Figure 3. This quadric surface is called an elliptic paraboloid.
Example: identifying traces of quadric surfaces
Describe the traces of the elliptic paraboloid [latex]x^2+\frac{y^2}{2^2}=\frac{z}{5}[/latex].
Show Solution
To find the trace in the [latex]xy[/latex]-plane, set [latex]z=0[/latex]: [latex]x^2+\frac{y^2}{2^2}=0[/latex]. The trace in the plane [latex]z=0[/latex] is simply one point, the origin. Since a single point does not tell us what the shape is, we can move up the [latex]z[/latex]-axis to an arbitrary plane to find the shape of other traces of the figure.
The trace in plane [latex]z=5[/latex] is the graph of equation [latex]x^2+\frac{y^2}{2^2}=1[/latex], which is an ellipse. In the [latex]xz[/latex]-plane, the equation becomes [latex]z=5x^{2}[/latex]. The trace is a parabola in this plane and in any plane with the equation [latex]y=b[/latex].
In planes parallel to the [latex]yz[/latex]-plane, the traces are also parabolas, as we can see in the following figure.
Figure 4. (a) The paraboloid [latex]x^2+\frac{y^2}{2^2}=\frac{z}{5}[/latex]. (b) The trace in plane [latex]z=5[/latex]. (c) The trace in the [latex]xz[/latex]-plane. (d) The trace in the [latex]yz[/latex]-plane.
try it
A hyperboloid of one sheet is any surface that can be described with an equation of the form [latex]\frac{x^2}{a^2}+\frac{y^2}{b^2}-\frac{z^2}{c^2}=1[/latex]. Describe the traces of the hyperboloid of one sheet given by equation [latex]\frac{x^2}{3^2}+\frac{y^2}{2^2}-\frac{z^2}{5^2}=1[/latex].
Show Solution
The traces parallel to the [latex]xy[/latex]-plane are ellipses and the traces parallel to the [latex]xz[/latex]– and [latex]yz[/latex]-planes are hyperbolas. Specifically, the trace in the [latex]xy[/latex]-plane is ellipse [latex]\frac{x^2}{3^2}+\frac{y^2}{2^2}=1[/latex], the trace in the [latex]xz[/latex]-plane is hyperbola [latex]\frac{x^2}{3^2}-\frac{z^2}{5^2}=1[/latex], and the trace in the [latex]yz[/latex]-plane is hyperbola [latex]\frac{y^2}{2^2}-\frac{z^2}{5^2}=1[/latex] (see the following figure).
Figure 5. (a) The trace in the [latex]xy[/latex]-plane. (b) The trace in the [latex]xz[/latex]-plane. (c) The trace in the [latex]yz[/latex]-plane. (d) The hyperboloid [latex]\frac{x^2}{3^2}+\frac{y^2}{2^2}-\frac{z^2}{5^2}=1[/latex].
Watch the following video to see the worked solution to the above Try IT.
You can view the transcript for “CP 2.53” here (opens in new window).Hyperboloids of one sheet have some fascinating properties. For example, they can be constructed using straight lines, such as in the sculpture in Figure 6(a). In fact, cooling towers for nuclear power plants are often constructed in the shape of a hyperboloid. The builders are able to use straight steel beams in the construction, which makes the towers very strong while using relatively little material (Figure 6(b)).
Figure 6. (a) A sculpture in the shape of a hyperboloid can be constructed of straight lines. (b) Cooling towers for nuclear power plants are often built in the shape of a hyperboloid.
Example: chapter opener: Finding the focus of a parabolic reflector
Energy hitting the surface of a parabolic reflector is concentrated at the focal point of the reflector (Figure 7). If the surface of a parabolic reflector is described by equation [latex]\frac{x^2}{100}+\frac{y^2}{100}=\frac{z}{4}[/latex], where is the focal point of the reflector?
Figure 7. Energy reflects off of the parabolic reflector and is collected at the focal point. (credit: modification of CGP Grey, Wikimedia Commons)
Show Solution
Since [latex]z[/latex] is the first-power variable, the axis of the reflector corresponds to the [latex]z[/latex]-axis. The coefficients of [latex]x^{2}[/latex] and [latex]y^{2}[/latex] are equal, so the cross-section of the paraboloid perpendicular to the [latex]z[/latex]-axis is a circle. We can consider a trace in the [latex]xz[/latex]-plane or the [latex]yz[/latex]-plane; the result is the same. Setting [latex]y=0[/latex], the trace is a parabola opening up along the [latex]z[/latex]-axis, with standard equation [latex]x^{2}=4pz[/latex], where [latex]p[/latex] is the focal length of the parabola. In this case, this equation becomes [latex]x^2=100\cdot\frac{z}4=4pz[/latex] or [latex]25=4p[/latex]. So [latex]p[/latex] is [latex]6.25[/latex] m, which tells us that the focus of the paraboloid is [latex]6.25[/latex] m up the axis from the vertex. Because the vertex of this surface is the origin, the focal point is [latex](0, 0, 6.25)[/latex].
Seventeen standard quadric surfaces can be derived from the general equation
[latex]Ax^2+By^2+Cz^2+Dxy+Exz+Fyz+Gx+Hy+Jz+K=0[/latex].
The following figures summarizes the most important ones.
Figure 8. Characteristics of Common Quadratic Surfaces: Ellipsoid, Hyperboloid of One Sheet, Hyperboloid of Two Sheets.
Figure 9. Characteristics of Common Quadratic Surfaces: Elliptic Cone, Elliptic Paraboloid, Hyperbolic Paraboloid.
Example: identifying equations of quadric surfaces
Identify the surfaces represented by the given equations.
- [latex]16x^{2}+9y^{2}+16z^{2}=144[/latex]
- [latex]9x^{2}-18x+4y^{2}+16y-36z+25=0[/latex]
Show Solution
- The [latex]x[/latex], [latex]y[/latex], and [latex]z[/latex] terms are all squared, and are all positive, so this is probably an ellipsoid. However, let’s put the equation into the standard form for an ellipsoid just to be sure. We have[latex]16x^{2}+9y^{2}+16z^{2}=144[/latex].
Dividing through by [latex]144[/latex] gives
[latex]\frac{x^2}9+\frac{y^2}{16}+\frac{z^2}9=1[/latex].
So, this is, in fact, an ellipsoid, centered at the origin.
- We first notice that the [latex]z[/latex] term is raised only to the first power, so this is either an elliptic paraboloid or a hyperbolic paraboloid. We also note there are [latex]x[/latex] terms and [latex]y[/latex] terms that are not squared, so this quadric surface is not centered at the origin. We need to complete the square to put this equation in one of the standard forms. We have
[latex]\begin{aligned} 9x^2-18x+4y^2+16y-36z+25&= 0\\ 9x^2-18x+4y^2+16y+25&=36z \\ 9(x^2-2x)+4(y^2+4y)+25&=36z \\ 9(x^2-2x+1-1)+4(y^2+4y+4-4)+25&=36z \\ 9(x-1)^2-9+4(y+2)^2-16+25&=36z \\ 9(x-1)^2+4(y+2)^2&=36z \\ \frac{(x-1)^2}4+\frac{(y+2)^2}9&=z. \end{aligned}[/latex]
This is an elliptic paraboloid centered at [latex](1, 2, 0)[/latex].
try it
Identify the surface represented by equation [latex]9x^{2}+y^{2}-z^{2}+2z-10=0[/latex].
Show Solution
Hyperboloid of one sheet, centered at [latex](0, 0, 1)[/latex].