断面積を用いた体積の求積

Volume is found by integrating cross-sectional areas along an axis perpendicular to the slices. For solids with known cross-sectional areas that vary predictably, you set up an integral where the area function is integrated over the height or length of the solid. This approach works for any solid where cross-sections can be expressed mathematically, making it a powerful alternative to classical geometric formulas. Learn more about calculation of volume of solids by integration.

A tetrahedron with an equilateral triangle base tapers linearly from the apex to the base. When you slice it perpendicular to the height axis, each cross-section maintains the same shape as the base—an equilateral triangle—but at a smaller scale. This occurs because the tetrahedron's geometry ensures that all cross-sections parallel to the base are similar equilateral triangles.

The side length of each cross-sectional triangle scales proportionally to the distance from the vertex. Using similar triangles, if the base has side length a and the tetrahedron has height h, the side length at height y is (y/h) times a. This linear scaling relationship reflects how the tetrahedron's width increases uniformly as you move from the apex toward the base.

The area of an equilateral triangle equals (√3/4) times the square of its side length. For a tetrahedron cross-section at height y with side length (y/h)a, the area becomes (√3/4) times [(y/h)a]². This quadratic relationship in y shows how cross-sectional area increases as you move down the tetrahedron's height.

Surveyors measure cross-sectional areas at multiple locations along an irregular pile or earthwork, then integrate these areas to estimate total volume. This practical method provides accurate material quantities for gravel, soil, or other construction materials without needing classical geometric formulas. It's particularly useful for irregular shapes where standard volume calculations don't apply.

The volume of a tetrahedron is proportional to the product of its height h and the square of its base side length a. Integrating the cross-sectional areas from the apex to the base yields this relationship, confirming known geometric principles. This result demonstrates how integration of variable cross-sections recovers classical volume formulas.

Placing the origin at the vertex with the y-axis along the height simplifies the integration setup. At the vertex, the cross-sectional area is zero, and it increases smoothly to maximum at the base. This coordinate choice makes the area function easier to express mathematically and the integration bounds straightforward: from y = 0 to y = h.