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Problem 39 (Shadows) Can one use the Sun's rays to produce a plane shadow of a cube: (i) in the form of an equilateral triangle? (ii) in the form of a square? (iii) in the form of a pentagon? (iv) in the form of a regular hexagon? (v) in the form of a polygon with more than six sides?

Short Answer

Expert verified
A cube can project a square shadow, but not a shadow in the form of an equilateral triangle, pentagon, regular hexagon, or a polygon with more than six sides.

Step by step solution

01

Understanding Shadow Projection

When an object like a cube is lit by the Sun, it casts a shadow that can take various shapes depending on the angle of the Sun's rays. To determine the possible shapes of the shadow, we must consider how the cube can be oriented relative to the Sun's rays.
02

Analyzing Shadow Shape (i): Equilateral Triangle

For the shadow to be an equilateral triangle, the three shadow lines must be of equal length and meet at 60-degree angles. Upon analyzing the cube, no orientation of the cube allows three sides to project equal-length lines at 60 degrees. Therefore, the shadow of the cube cannot be an equilateral triangle.
03

Analyzing Shadow Shape (ii): Square

A square shadow is possible when a face of the cube is parallel to the Sun's rays, meaning the Sun is 'in front' of the face. Thus, a cube can cast a square shadow.
04

Analyzing Shadow Shape (iii): Pentagon

To create a pentagonal shadow, five edges of the cube need to contribute to the shadow. Achieving a five-sided shadow shape with one regular face impossible due to the symmetry of a cube. Therefore, it's not possible to cast a pentagon shadow.
05

Analyzing Shadow Shape (iv): Regular Hexagon

A regular hexagon as a shadow involves six equal edges and internal angles of 120 degrees. The cube's geometry doesn't allow for six such lines along its outline; thus, it can't project a hexagonal shadow on a flat surface.
06

Analyzing Shadow Shape (v): Polygon with More than Six Sides

A shadow with more than six sides would require complex outlines that the basic structure of a cube cannot form. The maximum number of sides achievable from any projection of a cube's edges are six, hence a shadow with more than six sides is not possible.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Geometry
Understanding geometry is crucial when analyzing how shadows are formed. Geometry deals with the properties and relations of points, lines, surfaces, and solids. In the case of a cube casting a shadow, it’s about understanding the three-dimensional shape projecting onto a two-dimensional plane.
A cube is a regular polyhedron with six faces, all of which are square. Each face meets another at a 90-degree angle. Depending on how a cube is positioned, the projection onto a surface changes, affecting the shadow’s shape. Geometry helps predict these projections and understand why certain shadows are possible or impossible. For instance, a square shadow can be seen if one face of the cube is directly parallel to the sunlight.
Overall, geometry allows us to visualize and calculate potential shapes a three-dimensional object can cast as shadows on a flat surface.
Cube
A cube is a simple yet intriguing 3D shape. It has 6 faces, each being a square, 12 edges, and 8 vertices.
The symmetry of a cube is quite fascinating. Each face of the cube connects at right angles, which often influences the type of shadow it casts. Understanding these properties helps us predict the shadows a cube may project.
When the Sun shines on a cube, the orientation of the cube in relation to the rays will determine the shape of its shadow. For example, if one face aligns squarely with the Sun's rays, it can project a perfect square. However, more complex shapes like an equilateral triangle or a hexagon are not possible due to the uniformity and symmetry of the cube's structure.
Light and Shadows
The interaction between light and objects results in shadows. The Sun’s rays are parallel, and when they hit an object like a cube, a shadow is cast. Shadows are basically silhouettes that depend on the object’s shape and the light source’s angle.
Shadows provide us with visual evidence of the Sun’s position. When analyzing shadows, consider factors such as the direction and intensity of light, and how the object is tilted. For our cube, projecting a shadow involves understanding how different cube orientations impact the resultant shadow shape.
  • The angle of the Sun affects the shadow’s size and shape.
  • Parallel rays from the Sun mean that unless the cube is oriented perfectly, the outline may distort.
These elements are crucial in determining that a cube, despite its simplicity, cannot cast certain desired shadow shapes without altering its fundamental orientation.
Mathematics Problem Solving
Problem-solving in mathematics requires a strategic approach, especially for puzzles involving geometric shapes and their shadows.
In the exercise of determining possible shadows of a cube, the process involves analyzing forms, making logical deductions, and eliminating impossibilities. For example, determining that a pentagon shadow is impossible for a cube involves visualizing the face alignments and recognizing that the cube’s geometry disallows it.
A systematic plan enhances problem-solving skills:
  • First, understand the problem, translating it from a verbal statement to a set of possible projections.
  • Next, visualize or sketch possibilities.
  • Finally, validate or refute potential outcomes based on geometric principles.
This method reinforces understanding while strengthening mathematical reasoning.

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Most popular questions from this chapter

Problem 28 (A folded triangle) The equilateral triangle \(\triangle A B C\) has sides of length \(1 \mathrm{~cm} . \quad D\) and \(E\) are points on the sides \(A B\) and \(A C\) respectively, such that folding \(\triangle A D E\) along \(D E\) folds the point \(A\) onto \(A^{\prime}\) which lies outside \(\triangle A B C\) What is the total perimeter of the region formed by the three single layered parts of the folded triangle (i.e. excluding the quadrilateral with a folded layer on top )?

(a) In the "24 game" you are given four numbers. Your job is to use each number once, and to combine the four numbers using any three of the four basic arithmetical operations - using the same operation more than once if you wish, and as many brackets as you like (but never concatenating different numbers, such as "3" and "4" to make " \(34 "\) ). If the given numbers are \(3,3,4,4,\) then one immediately sees \(3 \times 4+3 \times 4=24 .\) With 3,3,5 , 5 it may take a little longer, but is still fairly straightforward. However, you may find it more challenging to make 24 in this way: (i) using the four numbers 3,3,6,6 (ii) using the four numbers 3,3,7,7 (iii) using the four numbers 3,3,8,8 . (b) Suppose we restrict the numbers to be used each time to "four \(4 \mathrm{~s}\) " \((4,4,4,4),\) and change the goal from "make \(24 ",\) to "make each answer from \(0-10\) using exactly four \(4 \mathrm{~s} "\). (i) Which of the numbers \(0-10\) cannot be made? (ii) What if one is allowed to use squaring and square roots as well as the four basic operations? What is the first inaccessible integer? Calculating by turning the handle deterministically (as with addition, or multiplication, or multiplying out brackets, or differentiating) is a valuable skill. But such direct procedures are usually only the beginning. Using mathematics and solving problems generally depend on the corresponding inverse procedures \(-\) where a certain amount of juggling and insight is needed in order to work backwards (as with subtraction, or division, or factorisation, or integration). For example, in applications of calculus, the main challenge is to solve differential equations (an inverse problem) rather than to differentiate known functions. Problem 14 captures the spirit of this idea in the simplest possible context of arithmetic: the required answer is given, and we have to find how (or whether) that answer can be generated. We will meet more interesting examples of this kind throughout the rest of the collection.

Problem \(29(A+B=C)\) The 3 by 1 rectangle \(A D E H\) consists of three adjacent unit squares: \(A B G H, B C F G, C D E F\) left to right, with \(A\) in the top left corner. Prove that $$ \angle D A E+\angle D B E=\angle D C E $$

The 4 by 4 "multiplication table" below is completely familiar. \(\begin{array}{rrrr}1 & 2 & 3 & 4 \\ 2 & 4 & 6 & 8 \\ 3 & 6 & 9 & 12 \\ 4 & 8 & 12 & 16\end{array}\) What is the total of all the numbers in the 4 by 4 square? How should one write this answer in a way that makes the total obvious?

The three integers \(a=3, b=4, c=5\) in the Pythagorean triple (3,4,5) form an arithmetic progression: that is, \(c-b=b-a\). Find all Pythagorean triples \((a, b, c)\) which form an arithmetic progression \(-\) that is, for which \(c-b=b-a\)

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