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Chapter 4: Problem 3
Describe the surface defined by (a) \(z=0\) (b) \(z=-1\) (c) \(r=2, z=1\) (d) \(\theta=90^{\circ}, z=3\) (e) \(r=2,0 \leqslant z \leqslant 4\)
Short Answer
Expert verified
The surfaces defined by the equations are as follows: (a) a horizontal plane at the origin, (b) a horizontal plane 1 unit below the origin, (c) a circular line in three-dimensional space at z=1 with radius 2, (d) a vertical line in the yz-plane at a distance equal to the radius from the origin and at height z=3, (e) a cylindrical surface with radius 2 and height 4, extending from z=0 to z=4.
Step by step solution
01
Analyzing the equation z=0
The equation z=0 defines all points in the xy-plane where the z-coordinate is equal to zero. This corresponds to a horizontal plane at the height z=0, which is the origin level in the context of a three-dimensional Cartesian coordinate system.
02
Describing the surface z=-1
Similar to the previous part, z=-1 describes all points where the z-coordinate is equal to -1. This is another horizontal plane parallel to the xy-plane, but this one is located 1 unit below the origin.
03
Defining the cylindrical surface r=2, z=1
The equation r=2, z=1 in cylindrical coordinates specifies a circle of radius 2 in the xy-plane located at z=1. Extending this circle along the z-axis does not change its position; therefore, this equation represents a single circle in three-dimensional space, not a surface.
04
Interpreting the equation theta=90 degrees, z=3
The equation in cylindrical coordinates \(\theta=90^\circ, z=3\) specifies all points that have an angular coordinate of 90 degrees from the positive x-axis and are 3 units above the xy-plane. This defines a vertical line in the yz-plane at y-coordinate equal to the radius of the cylinder, and z-coordinate 3.
05
Determining the shape defined by r=2,0 ≤ z ≤ 4
Here, r=2 specifies a circle of radius 2 in the xy-plane, and the inequality 0 ≤ z ≤ 4 means that this circle extends vertically from z=0 up to z=4. The result is a cylindrical surface with radius 2 and height 4, centered around the z-axis.
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Cartesian Coordinate System
The Cartesian coordinate system is the cornerstone of modern mathematics and engineering, representing points in space through a set of numerical coordinates. Imagine the system as a three-dimensional grid, where every point can be located via three numbers: the x, y, and z coordinates. These numbers correspond to the distances along the axes, which are perpendicular to each other.
In the context of our textbook exercise, understanding the Cartesian coordinate system allows students to visualize the surface defined by equations such as z=0
, which represents a flat, two-dimensional expanse on the horizontal xy-plane - think of it as a floor at ground level. By grasping this system's basic principles, translating complex spatial information into a comprehensible form becomes far easier.
Three-Dimensional Space
Three-dimensional space is a vast concept, extending the familiar two-dimensional plane into a new depth - the z-axis. It's like adding the concept of height to the length and width of a flat shape, creating a world where objects have volume and can exist above or below each other.
For students tackling problems such as z=-1
or z=3
, recognizing that these values represent vertical positions in three-dimensional space allows them to visualize planes and lines not just stretching out infinitely, but also floating above or sunk below a certain reference level. Spatial thinking is crucial here, as it helps students to mentally 'see' the geometric shapes dictated by mathematical equations.
Horizontal Plane
In geometry, a horizontal plane is a flat surface that runs left-to-right and front-to-back, but does not tilt up or down. Think of the surface of a perfectly still lake, undisturbed by any waves. In our textbook example, when the equation states z=0
or z=-1
, it defines a horizontal plane at different elevations in three-dimensional space. The concept of a horizontal plane also helps in understanding the background of cylindrical coordinates, dovetailing with the idea that every point can be plotted not just by its left-right or front-back position, but also by its height.
Cylindrical Surface
Cylindrical surfaces in mathematics can be tricky to envision, but a simple analogy is a soup can's outer surface. It's a three-dimensional shape, but you can roll it out into a flat rectangle. In cylindrical coordinates, we define surfaces using radius (r)
, angle (θ)
, and height (z)
.
Take the problem r=2, 0 ≤ z ≤ 4
: it tells us we have a cylinder with a fixed radius, starting at ground level (z=0)
and rising up to (z=4)
. By understanding cylindrical surfaces, students can better grasp the complexity of shapes described by cylindrical coordinates, which goes beyond the scope of rectangles and circles within a plane.
Mathematical Equations
Mathematical equations are more than strings of numbers and symbols; they are the language through which we describe shapes, positions, and relationships in space. Each equation in our exercise corresponds to a specific geometric entity. For instance, θ=90°, z=3
is not merely a pair of values but a vertical line situated at a specific orientation and elevation.
By learning to decode these equations, students can transition from abstract concepts to visualizing the concrete elements they represent. It's important not only to work through these equations step by step but also to understand what they convey about the spatial relationships they describe—that's where true mathematical comprehension lies.
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