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All questions of Rotation of shapes for Mechanical Engineering Exam

Consider a square sheet of side 1 unit. In the first step, it is cut along the main diagonal to get two triangles. In the next step, one of the cut triangles is revolved about its short edge to form a solid cone. The volume of the resulting cone, in cubic units, is ________
  • a)
    π/3
  • b)
    2π/3
  • c)
    3π/3
  • d)
Correct answer is option 'A'. Can you explain this answer?

Understanding the Problem
To find the volume of the cone formed by revolving one of the triangles, we first need to analyze the shape and dimensions of the triangles.
Step 1: Triangle Dimensions
- The square has a side length of 1 unit.
- Cutting it along the main diagonal divides the square into two right-angled triangles.
- Each triangle has a base and height of 1 unit.
Step 2: Selecting the Triangle
- We can choose either of the two triangles; the result will be the same.
- For the purpose of this explanation, let's select the triangle with vertices at (0, 0), (1, 0), and (0, 1).
Step 3: Revolving the Triangle
- When we revolve this triangle about its short edge (the edge along the y-axis, from (0, 0) to (0, 1)), we create a cone.
- The height of the cone is the length of the edge we are revolving around, which is 1 unit.
- The radius of the cone is the length of the base of the triangle, which is also 1 unit.
Step 4: Volume of the Cone
- The formula for the volume of a cone is given by:
Volume = (1/3) * π * r^2 * h
- Substituting the values:
- r = 1 unit (radius)
- h = 1 unit (height)
- Thus, the volume becomes:
Volume = (1/3) * π * (1^2) * 1 = (1/3) * π = π/3 cubic units.
Final Answer
The volume of the resulting cone is π/3 cubic units, which corresponds to option 'A'.

We have 2 rectangular sheets of paper. M and N, of dimensions 6 cm x 1 cm each. Sheet M is rolled to from an open cylinder by bringing the short edges of the sheet together. Sheet N is cut into equal patches and assembled to from the largest closed cube. Assuming the ends of the cylinder are closed, the ratio of the volume of the cylinder to that cube is ______
  • a)
    3/π
  • b)
    9/π
  • c)
    π/2
  • d)
Correct answer is option 'B'. Can you explain this answer?

Crack Gate answered
Given:
Two rectangular sheets M and N have the dimension 6 cm × 1 cm each,  in which cylinder will be formed by bringing the short edges of the sheet together. Given that ends of the cylinder are closed.
Concept:
Volume of cylinder = π x r2 x h
The volume of the cube = a3
Calculation:
Let be assume the side of the cube is a,
⇒ The area of the sheet N = 6 x a2
⇒ 6 × 1 = 6 x a2
⇒ a = 1
⇒ Volume of the cube = a3 = 13 = 1
⇒  The circumference  = 6
⇒ 2 × π × r = 6, r = 3/π and height of the cylinder = 1 cm
⇒ Volume of the cylinder = π x (3/π)2 x 1 = 9/π 
⇒ The required ratio = 9/π : 1
∴ The required result will be 9/π .

The least number of squares that must be added so that the line P-Q becomes the line of symmetry is ________.
  • a)
    4
  • b)
    3
  • c)
    6
  • d)
    7
Correct answer is option 'C'. Can you explain this answer?

Crack Gate answered
For symmetry we have to add some squares
After combining the obtained figure we get 

Hence for making the P-Q line symmetry 6 squares must be added

Corners are cut from an equilateral triangle to produce a regular convex hexagon as shown in the figure above. The ratio of the area of the regular convex hexagon to the area of the original equilateral triangle is
  • a)
    2 : 3
  • b)
    3 : 4
  • c)
    4 : 5
  • d)
    5 : 6
Correct answer is option 'A'. Can you explain this answer?

Gate Funda answered
Concept:
Area of an equilateral triangle  

where a = side of the triangle.
Calculation:
Given:
Let the side of the large triangle is 'a' then the side of the regular hexagon is a/3
If each triangle's area 
then the area of regular hexagon 
∴ The ratio of the area of the regular convex hexagon to the area of the original equilateral triangle is 2/3

Consider a cube made by folding a single sheet of paper of appropriate shape. The interior faces of the cube are all blank. However, the exterior faces that are not visible in the above view may not be blank.
Which one of the following represents a possible unfolding of the cube?
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'D'. Can you explain this answer?

Telecom Tuners answered
The Dark Shaded edge is perpendicular to a given line and the Light shaded edge is parallel to the given line so it can be assumed that option 4 is correct but there is no sign of + represented anywhere in the Question.
Hence, the correct answer is "Option 4".

Which one of the groups given below can be assembled to get the shape that is shown above using each piece only once without overlapping with each other? (rotation and translation operations may be used).
  • a)
  • b)
  • c)
  • d)
Correct answer is option 'B'. Can you explain this answer?

Telecom Tuners answered
option 1:

The above-given image will not be able to produce

The above-given image will be able to produce

as three triangles will make the bottom part whereas the remaining two will make the above part.
Option 3:

The above-given image will not be able to produce

as there is no scope for trapezium to fit in it.
Option 4:
The above-given image will not be able to produce

as one more triangle is required.
Thus option 2 is the correct  answer here.

Consider a square sheet of side 1 unit. The sheet is first folded along the main diagonal. This is followed by a fold along its line of symmetry. The resulting folded shape is again folded along its line of symmetry. The area of each face of the final folded shape, in square units, equal to _____.
  • a)
    1/4
  • b)
    1/8
  • c)
    1/32
  • d)
    1/16
Correct answer is option 'B'. Can you explain this answer?

Crack Gate answered
let us consider a square sheet of side 1 unit.
Now fold the sheet along the main diagonal.
Now fold this along their line of symmetry.
again fold this along their line of symmetry.
The side of the resulting shape = 1/2 unit
 Area of resultant shape = 1/2 x side2
Area of resultant shape= 1/2 x 1/2 x 1/2
Area of resultant shape = 1/8

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