Moment of Inertia Spinner

by cjbruce, published

Moment of Inertia Spinner by cjbruce Nov 15, 2017

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241Views 39Downloads Found in Physics & Astronomy


In our school, the equipment we use for rotational motion labs is over $200 for a single lab setup. The Moment of Inertia Spinner is designed to replace the expensive equipment with something that is much cheaper and easier for students to set up.

The entire lab setup for two students is less than $1, and the spinners are robust enough that they shouldn't break even if they are dropped.


Overview and Background

Moment of Inertia Spinner in action in the lab

The video above shows a typical lab setup of the Moment of Inertia Spinner.

Lesson Plan and Activity

Computing Moment of Inertia (45 minutes)

  1. Record video of the falling mass using Hudl Technique, or any other Video Motion Analysis software.
  2. Compute the angular acceleration of the stick, assuming a constant angular acceleration.
  3. Compute the torque applied to the spindle, given the weight of the hanging mass and the radius of the spindle (0.016 meters).
  4. Use the relationship between net torque, moment of inertia, and angular acceleration to compute the moment of inertia of the meterstick.
  5. How does the computed moment of inertia of the meterstick compare to the theoretical moment of inertia of a meterstick (I = 1/12ML²)? Why are they different?

Materials Needed


  • this spinner, printed at 1:1 scale
  • 2 x standard 8 mm ID x 22 mm OD skateboard bearings
  • For Best Fit: 5/8" wooden dowel
  • For A Poor Fit: a pencil
  • a meterstick or similar object to mount to the spinner to provide a large moment of inertia
  • tape to attach the meter stick to the spinner
  • 1 meter of string
  • a small hanging mass (500 grams or less is preferable)

Assembly Instructions

  1. Press fit one bearing into the top of the spinner.
  2. Press fit a second bearing into the bottom of the spinner.
  3. Insert wooden dowel or pencil through the spinner so that it penetrates both bearings.
  4. Tape a meterstick or other object onto the flat "wings" of the spinner.

Using the Spinner

Starting with a spinner that is assembled per the instructions above:

  1. Tie a small mass (100 - 500 grams is preferable) to a 1 meter long string.
  2. Wind the string around the spindle.
  3. Clamp or hold the dowel onto the table such that the meter stick can spin freely.
  4. Release the mass, recording the length of time it takes the mass to fall and the final rotational speed of the spinner.***

***We have found that this works well if students use their mobile devices with an app such as Hudl's "Technique" for iOS so that they can get accurate frame-by-frame video.


String Spindle Diameter - 32 mm
Bearing Outside Diameter - 22 mm
Bearing Inside Diameter - 8 mm
Maximum Bearing Width - 10 mm

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