Seesaw

SeeSaw on a circular arc

Point D oscillates on line AB, D being the projection of a point moving on the circle (E, EA). This circle is translated by AD, following the movement of D. On this moving circle (F, FD) we draw a fixed arc (GH) with central-angle 2 (radians). The formula calculates the central angle angle(DFG) in radians, of the moving circle, between the vertical radius and the radius to the right end of the arc. This gives the correct position of the right end G of the fixed arc, which placed there gives the impression of a seesaw.
Since we need signed distances, we use the tool [Measures\Length-Gauge] for the signed length of AD. The same tool is used to measure the distance AC. The two orthogonal lines are attached to a [Fixed-right-angle], so that their units are equal.

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