The circular disc and axle are mounted with the axle vertical between pivots, one on the base and one on a radial brace from a vertical pillar. The apparatus consists essentially of a low inertia circular disc mounted onto a vertical axle that can be rotated under the action of a falling mass. The upper surface of the circular disc is scribed with circles of 100 mm, 150 mm, 200 mm and 250 mm diameter and two holes are provided on each circle as sites to locate additional inertia discs. The low inertia circular disc, which is 300 mm diameter and 50.8 mm thick, is manufactured from plywood and is keyed to a 12 mm diameter by 305 mm long stainless steel axle. A 50 mm diameter pulley is attached to the axle.
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