Blackboard Ellipse Maker
Purpose
Draws precise ellipses on the blackboard using a string-and-pulley mechanism that enforces the constant-distance-sum definition. Illustrates Kepler's First Law and shows how changing the focus separation changes eccentricity.
Figure 1:
Several ellipses drawn with increasing focus separation. Two magnetic pulleys mark the foci; a string loop and chalk-holding pulley enforce the constant-distance-sum constraint.