The equipotential surface to which the plumb line is perpendicular at every point is called the what?

• A. Ellipsoid
• B. Geoid
• C. Sphere
• D. Manifold

Answer: (B)

Gravity is the force that defines the direction of the plumb line, and it acts perpendicular to surfaces of constant gravitational potential, called equipotential surfaces. The surface that would be everywhere perpendicular to the plumb line—i.e., everywhere normal to the gravity direction and that corresponds to the mean level of the oceans extended under the continents—is the geoid. In geodesy, the geoid represents this true equipotential surface of the Earth's gravity field and serves as the physical reference for measuring elevations. The other options describe smooth mathematical shapes or generic terms not tied to the gravity field's equipotential behavior—an ellipsoid is a smooth approximation, a sphere is a highly idealized constant-radius surface, and a manifold is a general mathematical concept not specific to Earth's gravity surface.