UNITY PHYSICS MASTER CLASS

The Parable of the Inertia Tensor

You're looking at Rigidbody.inertiaTensor. It sounds complex, but it answers a simple question:

"We know how hard it is to PUSH this object (Mass).
But how hard is it to SPIN this object?"

1. The Ice Skater Principle (Distance²)

In rotational physics, Resistance = Mass × Distance². Moving mass slightly outwards makes it drastically harder to spin.

Experiment A: 2D Torque
Angular Velocity
80
50
Inertia (I) = 0

2. Why is it a "Tensor" (Vector3)?

In 3D, difficulty changes depending on the axis. A long brick is easy to roll (longways) but hard to spin (flat).

Experiment B: The Shape Shifter
— Vel X
— Vel Z
1.0
3.5
0.5
20

3. The Secret Rotation

You asked: "Does the Tensor rotate opposite to the world?" No.

Unity finds the Principal Axes—the unique angle where the mass is perfectly balanced—and stores this offset in inertiaTensorRotation.

Observe the L-Shape below. The Object Local Axis (White) is aligned with the mesh, but the Principal Axes (RGB) rotate to balance the mass.

Experiment C: Principal Axes Visualizer

White = Transform.rotation
RGB = Inertia Principal Axes

4. Mass vs. Locked (The Graph Lab)

Experiment D: Locking Lab
━ Current
━ History
50
200