How often, when you watched or read a brushless motor review you've heard a sentence like "this motor will not have any problem spinning a heavy propeller" or "this motor is designed for light propellers". Or "this is a very light propeller". But what does that really mean?

Thanks to our good friend Sir Isaac Newton we know that

F = ma

Where F is the force, a is acceleration and m is the mass of an object. In this case, mass is the unit of inertia. This is all true for propellers if you want to throw them in linear motion. But propellers rotate and that changes a few things. Instead of mass we are interested in the moment of inertia and mass distribution around the rotation point.