Plumb bob
A weighted object suspended from a line that establishes a vertical reference by aligning with the direction of gravity.
Gravity: The attraction between masses, described in modern physics as the curvature of spacetime or as a force in Newtonian mechanics. The bob settles along the local direction of gravity.
Surveying: The measurement and mapping of land, boundaries, and built features. Surveyors use plumb references to transfer points and check instrument alignment.
Vertical direction: The direction locally aligned with gravity, perpendicular to a level surface. A plumb bob is a direct physical indicator of this direction.
Optical plummet: A surveying instrument that projects or views a point along a vertical optical axis. It centers instruments without a hanging line or settling bob.
Tension (physics): The pulling force transmitted through a taut rope, cord, or similar connector. Tension in the line supports the bob while gravity pulls it downward.
Masonry: Construction using units such as stone, brick, or concrete block joined with mortar. A plumb line checks whether walls and corners rise vertically.
Horizontal plane: A plane perpendicular to the local vertical direction defined by gravity. The bob's line is perpendicular to a locally level plane.
Laser plummet: A device that projects a laser beam along an instrument's vertical axis to transfer points between elevations. Its beam provides a visible vertical reference without relying on a suspended weight.
Simple pendulum: An idealized mass suspended from a fixed point by a massless, inextensible line and free to swing. A plumb bob behaves like a pendulum while swinging toward rest.
Carpentry: The craft of cutting, joining, and shaping wood for structures and objects. Carpenters use a plumb bob to align posts, frames, and other upright members.