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# What are the moments of inertia of the disks included in the Rotary Motion Accessory Kit?

The Rotational Motion Accessory Kit, order code AK-RMV, \$110 includes several discs for rotary motion experiments.

Both smooth discs have a moment of 1.05 x 10^-4 kg m^2 to three decimal places.
The disc with a small hole is 106.1 g, 89.0 mm diameter, 3.6 mm hole.
The disc with cork and a larger hole is 105.2 g, 89.0 mm diameter, 6.0 mm hole. These dimensions and masses are typical.

The heavy washer values are approximate, as dimensions and mass vary:

The sample we measured was 267.6 g, 88.5 mm diameter, 27 mm diameter inner hole, for a moment of 2.86 x 10^-4 kg m^2

The plastic center disk that fits the washer is just 4.5 g in mass, so its approximate moment is 5 x 10^-7 kg m^2.

The three step pulley has an approximate moment of 2 x 10^-6 kg m^2. See also TIL 1012: What are the dimensions of the 3-step pulley/wheel on the Rotary Motion Sensor?.

In comparison to the metal disks, the plastic parts have negligible moments and can be disregarded.

Note: In early editions, experiment 14 of the Advanced Physics with Vernier — Mechanics, order code PHYS-AM, \$48 book incorrectly listed the value for the Moment of inertia as 100 times too large.