Similar Solids and Cavalieri's Principle — Worked Examples

These examples develop the square-cube law and Cavalieri reasoning from direct scaling to inverse and engineering applications.

Example 1: Linear, area, and volume scale factors

A geometric model is enlarged so every corresponding length is multiplied by 1.501.50. Determine the corresponding area and volume multipliers.

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Example 2: Volume of a similar storage vessel

A prototype vessel has capacity 2.40 m32.40\,\text{m}^3. A geometrically similar vessel is built at twice every internal linear dimension. Determine its capacity.

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Example 3: Recover a linear scale factor from area ratio

Two similar solids have corresponding surface areas 54.0 cm254.0\,\text{cm}^2 and 150 cm2150\,\text{cm}^2. Find the linear scale factor from the smaller to the larger solid.

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Example 4: Recover a dimension from volume ratio

Two similar concrete specimens have volumes 0.0640 m30.0640\,\text{m}^3 and 0.216 m30.216\,\text{m}^3. A corresponding edge of the smaller specimen is 0.800 m0.800\,\text{m}. Determine the matching edge of the larger specimen.

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Example 5: Mass scaling for the same material

A solid bronze model has mass 18.0 kg18.0\,\text{kg}. A geometrically similar model of the same bronze is made with every dimension 0.6000.600 times the original. Determine its mass.

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Example 6: Similar geometry but different material density

A steel model is geometrically similar to an aluminum prototype at linear scale factor k=0.500k=0.500. Let the steel density be 7850 kg/m37850\,\text{kg/m}^3 and aluminum density be 2700 kg/m32700\,\text{kg/m}^3. Determine the steel-to-aluminum mass ratio.

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Example 7: Right and oblique prism volumes

A right prism and an oblique prism have identical base areas of 12.5 m212.5\,\text{m}^2 and the same perpendicular height of 7.20 m7.20\,\text{m}. The oblique prism has lateral edges 8.60 m8.60\,\text{m} long. Compare their volumes.

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Example 8: Same base and height do not guarantee equal volume

A prism and a pyramid have the same base area B=30.0 m2B=30.0\,\text{m}^2 and the same perpendicular height h=6.00 mh=6.00\,\text{m}. Show why these data alone do not satisfy Cavalieri's conclusion.

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Example 9: Similar section inside a cone

A cone is 18.0 m18.0\,\text{m} high with base area 254 m2254\,\text{m}^2. Determine the area of a section parallel to the base located 12.0 m12.0\,\text{m} from the apex.

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Example 10: Volume fraction of an apex pyramid

A plane parallel to the base of a pyramid is located at 40.0%40.0\% of the full height measured from the apex. What fraction of the full pyramid volume lies between the apex and this plane?

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Example 11: Surface-area-to-volume ratio under enlargement

A family of similar solids is enlarged by a linear factor k=2.50k=2.50. Determine the factor by which its surface-area-to-volume ratio changes.

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Example 12: Non-uniform coordinate scaling

A rectangular solid has original volume 48.0 m348.0\,\text{m}^3. Its xx, yy, and zz dimensions are independently scaled by 1.201.20, 0.8000.800, and 1.501.50, respectively. Determine the new volume and state whether the new solid is necessarily similar to the original.

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Scaling check

Always define which solid is the reference before computing kk. Reversing the scale-factor direction reverses every derived ratio. For Cavalieri comparisons, equality at only one cross-section is insufficient; the cross-sectional areas must match at every corresponding level.