Fractals and Visualising Solids | IT

Question 12

Draw a net with appropriate measurements that can be folded into a regular tetrahedron. Verify if it works by making an actual cutout.

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Solution

A net for a regular tetrahedron is made of four equilateral triangles joined together.

Step 1 — Understanding a Regular Tetrahedron

A regular tetrahedron is a special 3D shape. It has four faces, which are its flat surfaces. Each face is an equilateral triangle. This means all sides of each face are equal. Also, all the edges of the tetrahedron itself are equal in length.

Step 2 — Designing the Net

To make a net, we imagine unfolding the tetrahedron flat onto a surface. If we choose one equilateral triangle as the base, the other three faces must attach to its sides. When we fold these three outer triangles upwards, they will meet at a single point. This point forms the top corner, or apex, of the tetrahedron. The most common net for a regular tetrahedron looks like one large equilateral triangle. This large triangle is divided into four smaller, identical equilateral triangles.

Step 3 — Choosing Measurements

We need to pick a suitable size for our net. Let's choose a size that is easy to draw and cut out. We will make the side length of the final tetrahedron 3 cm. This means each of the four small equilateral triangles in our net will have sides of 3 cm. So, the large equilateral triangle that forms the entire net will have a side length of 2×3 cm2 \times 3 \text{ cm}.

2×3 cm2 \times 3 \text{ cm}

6 cm\boxed{6 \text{ cm}}

Therefore, the large equilateral triangle we draw must have a side length of 6 cm.

Step 4 — Drawing the Net

First, draw an equilateral triangle. Make sure each side of this triangle is 6 cm long. Use a ruler to measure the sides accurately. Use a protractor to ensure each angle is 60 degrees. Next, find the midpoint of each of the three sides of this large triangle. Mark these midpoints clearly with a pencil. Now, connect these three midpoints using dotted lines. These dotted lines show where you will fold the paper. The outer edges of the large 6 cm triangle are where you will cut the paper. This drawing is the complete net for the regular tetrahedron.

Diagram 1

Step 5 — Verifying the Net

Carefully cut out the large equilateral triangle along its solid outer edges. Now, gently fold the paper upwards along the three dotted lines (DE, EF, and FD). You will see that the three outer triangles rise up and meet perfectly at a single point. This forms a 3D regular tetrahedron. Each edge of this tetrahedron will measure 3 cm long.

Answer

Draw an equilateral triangle with a side length of 6 cm. Mark the midpoints of each side of this triangle. Connect these midpoints using dotted lines. The outer edges of the 6 cm triangle are the cut lines. The dotted lines are the fold lines. When folded, this net forms a regular tetrahedron with each edge measuring 3 cm. To verify, cut out the net and fold it along the dotted lines.

More questions in IT

Q1

Draw the initial few steps (at least till Step 2) of the shape sequence that leads to the Sierpinski Carpet.

By its construction, each step in the sequence has (i) squares of the same size that remain in the figure, and the size of these squares becomes smaller and smaller as the step number increases, and (ii) square holes that are formed by removing square pieces.

Q2

Show that by joining the midpoints of an equilateral triangle, we divide it into 4 identical equilateral triangles.

[Hint: Note that the corner triangles are isosceles.]

This fractal is called the Sierpinski Triangle/Gasket.

Q3

In previous classes, you've seen solids that are much simpler than an elephant or cat, such as cubes, spheres, cylinders, and cones. What would the profiles of these look like, from different viewpoints?

Q4

Can you describe a solid and a viewpoint that would result in each of the following cases? If it helps, you can imagine the solid passing through a wall like Tom did, and leaving a hole of the appropriate shape.

  1. A solid whose profile has a square outline
  2. A solid whose profile has a circular outline
  3. A solid whose profile has a triangular outline
Q5

As we saw with the elephant, a given solid might have very different profiles from different viewpoints. Can you visualise solids that have the following contrasting profiles?

Spend some time on this, and if you are finding it difficult to visualise, you may look around and use objects that are around you, or that you will make in the next section. Feel free to consider viewpoints from any direction, including directly above the object.

  1. A solid with a rectangular profile from one viewpoint and a circular profile from another viewpoint
  2. A solid with a circular profile from one viewpoint and a triangular one from another viewpoint
  3. A solid with a rectangular profile from one viewpoint and a triangular one from another viewpoint
  4. A solid with a trapezium shaped profile from one viewpoint and a circular one from another viewpoint
  5. A solid with a pentagonal profile from one viewpoint and a rectangular one from another viewpoint

Are there unique solids for each of the conditions, or can you come up with multiple possibilities?

Q6

If the congruent polygons of a prism have 10 sides, how many faces, edges and vertices does the prism have? What if the polygons have nn sides?

Q7

If the base of a pyramid has 10 sides, how many faces, edges and vertices does the pyramid have? What if the base is an nn-sided polygon?

Q8

What is a net of a cube?

Q9

Visualise how it can be folded to form a cube.

Q10

What is a net of a regular tetrahedron? Which of the following are nets of a regular tetrahedron?

Q11

Are there any other possible nets?

Q12

Draw a net with appropriate measurements that can be folded into a regular tetrahedron. Verify if it works by making an actual cutout.

Q13

Draw a net with appropriate measurements that can be folded into a square pyramid. Verify if it works by making an actual cutout.

Q14

What is the net of a cylinder?

If the circular faces of a cylinder are unfolded, and if a cut is made along the height of the cylinder, as shown in the figure below, then we get

Q15

What are the sidelengths of the rectangle obtained?

Q16

How will the net of a cone look?

Q17

If the cone is slit open along the line ll and then unrolled, what will we get?

Observe that all the points on the boundary of the base circle are at equal distances from 'O'. So after unrolling the cone, the boundary of the net will be a portion of a circle with centre O.

Q18

What surface do you construct by using the above net, in which O is not the centre of the boundary circle? Make a physical model to help you answer this question!

Q19

Draw a net with appropriate measurements that can be folded into a triangular prism. Verify that it works by making an actual cutout.

Q20

Taking all the triangles in the net to be equilateral, make a cutout of the net and fold it to form an octahedron.

Q21

What is the shortest path for the ant to reach the laddu?

Q22

What about in the following case?

Q23

If we think that a certain path is the shortest, how can we be sure that it truly is, among all the infinite possibilities?

Q24

For example, are either of these the shortest path?

Q25

What does this show?

Q26

Have we now completely analysed the problem of finding the shortest path between two points on a cuboid?

Q27

What is the length of the shortest path between the ant and the laddu?

Q29

What happens to the length of a line in its projection?

Q30

Can you now compare the lengths pp and ll?

Q31

When is the length of the projected line equal to its actual length?

Q32

What do you think are the different possible projections of a square that we get based on its orientation?

Q33

What do you think is the projection of a parallelogram under different orientations?

Can this ever be a quadrilateral that is not a parallelogram? As a starting point, you could think about the projection of a pair of parallel lines.

Q34

What can you say about the projection of an nn-sided regular polygon?

[Hint: Projection of a polygon is composed of the projections of its sides.]

Q35

How would the projections of a cube and a cone look?

Q36

See Figures 4.2–4.5. In each case, see if you can visualise another object that gives the same projection.

Q37

Find another object that makes the same projection as that of a given cone.

Q39

Have you played Tetris? There are five basic shapes in Tetris, corresponding to the different ways of arranging four squares.

Q40

Context: In Fig. 4.8, there are five basic shapes in Tetris, corresponding to the different ways of arranging four squares.

Q. Imagine these are cubes, not squares. Draw each of these on your isometric paper (you can find it at the end of the book).

Q41

Why is this correspondence between directions on isometric paper and axes of the solid so effective for communicating the shape of the solid?

Q42

Can you try drawing the other tetris shapes on isometric paper?

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