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Question

The diameter of a copper solid sphere is 6 cm. The sphere is melted and recast into a wire. If the diameter of the wire is 0.5 cm, then what is the length of the wire ?

This question was previously asked in
CDS 1 2026 Maths Question Paper (12-Apr-2026)
The correct answer is
5.76 m

Calculating Wire Length from Melted Sphere Volume

The core principle is that the volume of the material remains constant when a solid is melted and recast. Therefore, the volume of the copper sphere is equal to the volume of the wire (which is a cylinder).

Sphere Calculations

  • Given sphere diameter = 6 cm.
  • Sphere radius (\(r_s\)) = Diameter / 2 = 6 cm / 2 = 3 cm.
  • Volume of a sphere (\(V_s\)) is given by the formula:

    \(V_s = \frac{4}{3} \pi r_s^3\)

  • Substituting the radius:

    \(V_s = \frac{4}{3} \pi (3 \text{ cm})^3 = \frac{4}{3} \pi (27 \text{ cm}^3) = 36 \pi \text{ cm}^3\)

Wire Calculations

  • Given wire diameter = 0.5 cm.
  • Wire radius (\(r_w\)) = Diameter / 2 = 0.5 cm / 2 = 0.25 cm.
  • The wire is cylindrical. Volume of a cylinder (\(V_w\)) is given by:

    \(V_w = \pi r_w^2 h\)

    where '\(h\)' is the length of the wire.
  • Substituting the radius:

    \(V_w = \pi (0.25 \text{ cm})^2 h = \pi (0.0625 \text{ cm}^2) h\)

Equating Volumes and Finding Length

  • Set the volume of the sphere equal to the volume of the wire:

    \(V_s = V_w\)

    \(36 \pi \text{ cm}^3 = \pi (0.0625 \text{ cm}^2) h\)

  • Cancel \(\pi\) from both sides:

    \(36 \text{ cm}^3 = 0.0625 \text{ cm}^2 \times h\)

  • Solve for '\(h\)':

    \(h = \frac{36 \text{ cm}^3}{0.0625 \text{ cm}^2}\)

    \(h = 576 \text{ cm}\)

  • Convert the length from centimeters to meters (since 1 m = 100 cm):

    \(h = \frac{576}{100} \text{ m} = 5.76 \text{ m}\)

The length of the wire is 5.76 m.

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Important Questions from 3-D Mensuration

  1. Find the total surface area of a closed cylinder having a base radius of 70 m and a height of 110 m. [Use π = \(22\over7\)]

  2. Which of the following is a geometrical figure with a three-dimensional geometry that has eight vertices and six rectangular faces?

  3. The curved surface area of a cylinder is half of its total surface area. If its height is 195 cm, then find its diameter (in cm).
  4. The diameter of the base and slant height of a right circular cone are 30 cm and 113 cm, respectively. Find the volume (in cm³) of the given cone.
    (Use $\pi = \frac{22}{7}$)

  5. A cylindrical rod has an curved surface area of $4,900 \text{ cm}^2$. If the length of the rod is 97 cm, then the radius (in cm) of the rod, correct to two places of decimal, is:
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