Skill 4 of 10
In Progress


Stuff is “tough” to move around. We called this inertia earlier.1

  • “Throwing toughness”, or we could say linear inertia,2 when throwing a heavy stone or so, we called mass.
  • Stuff can also be tough to spin or rotate, like a heavy flywheel. What should we call such “rotational toughness”, or angular inertia?

We could call it ‘angular mass’, and many would simply not call it anything but keep saying ‘inertial mass’.[1,2] But in scientific circles it is most commonly called moment-of-inertia.3 Let’s give it the symbol $I$.4

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  1. Dynamics of Rotational Motion: Rotational Inertia’ (web page), BC Open Textbooks, OpenStaxCollege, 2012, (accessed Feb. 6th, 2020)
  2. Dynamics of Rotational Motion: Rotational Inertia’ (web page), Khan Academy, 2008, (accessed Feb. 6th, 2020)
  3. Online Etymology Dictionary’ (dictionary), Douglas Harper,
  4. The Science of Mechanics’ (book), Ernst Mach, The Open Court Publishing Co., 4th ed., English translation, 1919, (pages 173-187)
  5. Theoria motus corporum solidorum seu rigidorum’ (book, in Latin), Leonhard Euler, A. E. Roser., 1765,, ISBN 978-1-4297-4281-8 (page 166)