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# Inertia Tensor properties

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Are 3x3 inertia tensors rotational only, or do they also affect other transformations? What I'm really trying to get at: I have a quaternion for angular momentum, and one for orientation... how do I apply the inverse of the inertia tensor? Should angular momentum be represented by a quaternion?

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Quote:
 Original post by coder0xffAre 3x3 inertia tensors rotational only, or do they also affect other transformations?What I'm really trying to get at: I have a quaternion for angular momentum, and one for orientation... how do I apply the inverse of the inertia tensor? Should angular momentum be represented by a quaternion?
Again, not really qualified to answer this question (see my other post), but I think a vector representation is preferred for angular velocity and momentum. The derivative of your orientation quat can be calculated directly from the angular velocity vector, and the angular velocity is derived from the angular momentum via the inertia tensor, which should be a 3x3 matrix (or just the diagonals, under the right circumstances). The inverse inertia tensor is usually applied to the angular momentum vector via a similarity transform, which allows the tensor to be stored in local space rather than be recomputed every update. And yes, I'm pretty sure the inertia tensor only relates to angular effects (the linear equivalent being the object's mass).

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If you think of mass as the resistance of a body against linear motion, the inertia tensor describes the resistance of a body against angular motion.

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For someone not qualifed, jyk is rather bang on :) Not only for this post, for your previous post also. I would drop the quaternion for the angular momentum, you'll be applying a force as a vector on your object and deriving the torque from that force also as a vector. Then your angularMomentum += vTorque * elapse_time. The vTorque here would be calculated from the cross product of the angular force and (incidentVector - centerPositionOfyourObject).

Good luck!
Shadx

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