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Question

Three containers, namely cylinder, square prism and triangular prism, are given. Which one of the following conditions will ensure that they have identical volume?

The correct answer is

Their base areas and heights are identical

Identical Volume for Different Prisms and Cylinder

The question asks for the condition under which a cylinder, a square prism, and a triangular prism will have identical volumes. To answer this, we need to understand how the volume of these shapes is calculated.

Volume Formula for Prisms and Cylinders

The volume of any prism or a cylinder is generally calculated using a simple formula:

Volume = Base Area $\times$ Height

Let's consider each shape mentioned in the question:

  • Cylinder: A cylinder can be thought of as a circular prism. Its base is a circle. If the radius of the base is \$r\$ and the height is \$h\$, the base area is \$\pi r^2\$. The volume of the cylinder is \$V_{cylinder} = (\pi r^2) \times h\$.
  • Square Prism: A square prism has a square base. If the side length of the square base is \$s\$ and the height is \$h\$, the base area is \$s^2\$. The volume of the square prism is \$V_{square\_prism} = (s^2) \times h\$.
  • Triangular Prism: A triangular prism has a triangular base. If the area of the triangular base is \$A_{base}\$ and the height is \$h\$, the volume of the triangular prism is \$V_{triangular\_prism} = A_{base} \times h\$.

Condition for Identical Volume

From the volume formulas above, we can see a common structure: Volume = Base Area $\times$ Height. For the volumes of the cylinder, square prism, and triangular prism to be identical, the product of their respective base area and height must be the same.

If we want their volumes to be identical, say \$V_{cylinder} = V_{square\_prism} = V_{triangular\_prism} = V\$, then based on the formula:

\[ V = (\text{Base Area of Cylinder}) \times (\text{Height of Cylinder}) \] \[ V = (\text{Base Area of Square Prism}) \times (\text{Height of Square Prism}) \] \[ V = (\text{Base Area of Triangular Prism}) \times (\text{Height of Triangular Prism}) \]

The simplest and most direct condition that ensures all three volumes are identical is when both the base areas AND the heights of all three containers are identical.

Let's assume the base area of the cylinder is \$A_1\$, the base area of the square prism is \$A_2\$, and the base area of the triangular prism is \$A_3\$. Let their respective heights be \$h_1, h_2, h_3\$. Their volumes are \$V_1 = A_1 h_1\$, \$V_2 = A_2 h_2\$, and \$V_3 = A_3 h_3\$.

For \$V_1 = V_2 = V_3\$, if we set \$A_1 = A_2 = A_3 = A\$ and \$h_1 = h_2 = h_3 = h\$, then \$V_1 = A \times h\$, \$V_2 = A \times h\$, and \$V_3 = A \times h\$. In this case, \$V_1 = V_2 = V_3\$.

Analyzing the Options

  • Option 1: Their base areas and heights are identical

    As shown above, if the base area and the height are the same for all three shapes, their volumes will be identical according to the formula Volume = Base Area $\times$ Height. This condition guarantees identical volumes.

  • Option 2: Their base areas are identical

    If only the base areas are identical (e.g., \$A_1 = A_2 = A_3 = A\$), but the heights are different (e.g., \$h_1 \neq h_2 \neq h_3\$), then the volumes would be \$V_1 = A h_1\$, \$V_2 = A h_2\$, and \$V_3 = A h_3\$. These volumes would only be identical if the heights happened to also be identical. So, this condition alone is not sufficient.

  • Option 3: Their heights are identical

    If only the heights are identical (e.g., \$h_1 = h_2 = h_3 = h\$), but the base areas are different (e.g., \$A_1 \neq A_2 \neq A_3\$), then the volumes would be \$V_1 = A_1 h\$, \$V_2 = A_2 h\$, and \$V_3 = A_3 h\$. These volumes would only be identical if the base areas happened to also be identical. So, this condition alone is not sufficient.

  • Option 4: Their base areas and heights are different

    If both base areas and heights are different, it is very unlikely that their volumes would be identical. While it might be possible for specific combinations of different base areas and heights to result in the same volume (e.g., \$A_1 h_1 = A_2 h_2\$), this is not a guaranteed condition based on them being different. This condition does not ensure identical volumes.

Therefore, the condition that will ensure they have identical volume is when their base areas and heights are identical.

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Important Questions from Solid Figures

  1. A cylindrical tube, open at both ends, is made of a metal sheet which is 0.5 cm thick. Its outer radius is 4 cm and length is 2 m. How much metal (in cm 3) has been used in making the tube?

  2. The volume of a right circular cone is 308 cm 3 and the radius of its base is 7 cm. What is the curved surface area (in cm 2) of the cone? (Take π =  \(\frac{22}{7} \) )

  3. The slant height and radius of a right circular cone are in the ratio 29 ∶ 20. If its volume is 4838.4 π cm 3, then its radius is: 

  4. Six cubes, each of edge 2 cm, are joined end to end. What is the total surface area of the resulting cuboid in cm 2?

  5. A solid cube of side 8 cm is dropped into a rectangular container of length 16 cm, breadth 8 cm and height 15 cm which is partly filled with water. If the cube is completely submerged, then the rise of water level (in cm) is:

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