Blocks Moment of Inertia is Wrong

The moment of inertia of blocks is about double what it should be.
The moment of inertia of spheres is correct, I am unaware of other objects.

When you make a block with a size of (x, y, z) and make a block of the same size out of many smaller blocks, you expect it to behave the same way, but it does not. The block made out of smaller blocks rotates faster when torqued the same way.

Also blocks look wrong when you drop them on the ground. Like they should spin faster.

I THINK the moment of inertia of blocks is computed as mass/6*(y^2 + z^2, z^2 + x^2, x^2 + y^2)
It SHOULD be mass/12*(y^2 + z^2, z^2 + x^2, x^2 + y^2)

place file: MomentComparison.rbxl (20.4 KB)


Just noticed that internally we use the “Hollow Block” moment of inertia instead of solid block. However we calculate mass based on the object being solid. This is probably why this happens.

Definitely a bug.


We’re finally starting the switch from hollow block to solid block inertia (and fixing CornerWedge/Wedge mass while we’re at it):