極座標の二重積分

Imagine a landscape architect designing a semicircular pond with radius L. To find the amount of concrete needed for the base, the pond's total area must be calculated.

Consider a rectangular area element dA with length dx and height dy inside the semicircular region S. The double integral in Cartesian coordinates gives the pond’s area.

For the semicircular boundary, the limits of integration involve a square root function. This makes the calculation difficult.

Polar coordinates help overcome this difficulty by describing each point in terms of a distance r from the center and an angle theta.

For this semicircle, S, r runs from zero to L, and theta runs from zero to pi.

Now, the area element dA becomes a circular sector. Here r accounts for the increasing area as the sectors widen away from the center.

The pond's area is given by double integrals in polar coordinates. Integrating r from zero to L and theta from zero to pi gives the total area of the pond.

This shows how polar coordinates simplify integrals over circular geometry.