Chapter 2: Problem 399
For the following exercises, the equation of a surface in rectangular coordinates is given. Find the equation of the surface in spherical coordinates. Identify the surface. $$x^{2}+y^{2}-3 z^{2}=0, z \neq 0$$
Short Answer
Expert verified
The surface is a double cone centered around the z-axis.
Step by step solution
01
Recall the Spherical Coordinates Formulas
In spherical coordinates, the formulas for converting from rectangular coordinates are: - \( x = \rho \sin \phi \cos \theta \)- \( y = \rho \sin \phi \sin \theta \)- \( z = \rho \cos \phi \)where \( \rho \) is the distance from the origin, \( \phi \) is the angle with respect to the positive z-axis, and \( \theta \) is the angle projected onto the xy-plane from the positive x-axis.
02
Substitute Rectangular to Spherical Formulas
Use the formulas to substitute \( x \), \( y \), and \( z \) in the equation \( x^2 + y^2 - 3z^2 = 0 \). This yields:\[(\rho \sin \phi \cos \theta)^2 + (\rho \sin \phi \sin \theta)^2 - 3(\rho \cos \phi)^2 = 0\]
03
Simplify the Equation
Simplify the equation by first combining like terms:\[ \rho^2 \sin^2 \phi (\cos^2 \theta + \sin^2 \theta) - 3\rho^2 \cos^2 \phi = 0 \]Since \( \cos^2 \theta + \sin^2 \theta = 1 \), this simplifies to:\[ \rho^2 \sin^2 \phi - 3\rho^2 \cos^2 \phi = 0\]
04
Factor and Solve the Equation
Factor out \( \rho^2 \) (assuming \( \rho eq 0 \)):\[\rho^2 (\sin^2 \phi - 3\cos^2 \phi) = 0\]Simplifying, solve for \( \phi \):\[\sin^2 \phi = 3\cos^2 \phi\]
05
Express as a Tangent Equation
Divide through by \( \cos^2 \phi \) (since \( \phi eq \frac{\pi}{2} \)), resulting in:\[\tan^2 \phi = 3\]
06
Identify the Surface
Since \( \tan \phi = \sqrt{3} \), this implies \( \phi = \frac{\pi}{3} \) or \( \phi = \frac{2\pi}{3} \). These angles imply that the surface is a double cone symmetrically centered around the z-axis.
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Rectangular Coordinates
Rectangular coordinates, often referred to as Cartesian coordinates, employ a three-axis system to describe the location of a point in space. These axes are commonly labeled as x, y, and z. Each point is defined by a triplet
- x: the position along the horizontal axis
- y: the position along the vertical axis
- z: the position along the depth axis (perpendicular to x and y)
Surface Identification
Identifying a surface in geometry involves recognizing the shape and features of an equation that describes its form. In our exercise, the equation presented is:\[x^{2}+y^{2}-3z^{2}=0\]To understand what this equation represents, let's break it down:
- The terms \(x^2 + y^2\) suggest a cylindrical shape since they touch zero with the z-component ignored.
- However, the presence of the term \(-3z^2\) modifies this perception. It suggests a subtraction related to z, indicating a surface symmetrical about the z-axis.
Coordinate Conversion
Coordinate conversion is a useful tool in mathematics, allowing navigation between different systems to simplify equations or reveal geometrical features. Converting from rectangular coordinates
- Using the formulas: \(x = \rho \sin \phi \cos \theta \), \(y = \rho \sin \phi \sin \theta \), \(z = \rho \cos \phi \)
- Here, \(\rho\) signifies the distance from the origin, \(\phi\) is the angle from the positive z-axis, and \(\theta\) is the angle in the xy-plane.
Conic Sections
Conic sections are curves generated by the intersection of a plane with a double-napped cone. This family of curves includes circles, ellipses, parabolas, and hyperbolas, each representing different scenarios based on the intersecting angle.
- A circle forms when the plane is perpendicular to the cone's axis.
- An ellipse results when the plane makes an oblique cut.
- A parabola occurs when the intersection is parallel to the edge.
- A hyperbola arises with steep angles intersecting both naps of the cone.