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# torques and rotations, summing

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Okay, I read a post, one guy said that even in quaternions, order matters(in relation to rotation), I read the Matrix and Quat FAQ and it seemed to suggest otherwise. Also, if I have a bunch of torques acting on a body, I can just add them up in any order I choose since torque is a vector for my example and then use that summation to rotate the body, correct?

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The order you multiply matrices and quaterninons matters. This corresponds to the fact that if you combine two rotations the result depends on which is done first.

But when adding vectors such as torque, or adding quaternions or matrices, the order does not matter.

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Thank you, johnb, and just to be sure, if I have two or more quats representing my rotations, to get the combined rotation I multiply them, correct?
quote:
This corresponds to the fact that if you combine two rotations the result depends on which is done first.

Yet if I have several torques(as vectors), I can add them and the order doesn''t matter. Isn''t that like combining rotations?

I feel that I am so close.

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>> This corresponds to the fact that if you combine two
>> rotations the result depends on which is done first.

> Yet if I have several torques(as vectors), I can add them
> and the order doesn''t matter. Isn''t that like combining
> rotations?

A rotation is a turn of an object in space. E.g. "90 degrees clockwise around the x axis" is a rotation.

A torque is a measure of rotational pressure, caused by a force applied off-centre to a body.

If you apply a torque you may get a rotation but they are different things, decribed using different tools (vectors for torque, quaternions or matrices for rotations) and their mathematical rules are quite different.

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I see, quats and matrices describe a rotation explicitly, while a torque *may* produce a rotation. Thank you.

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Torque is a force, so it''s described by a vector.

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quote:
Original post by Zipster
Torque is a force, so it''s described by a vector.

No. A torque is a torque and a force is a force. They aren''t the same thing.

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Yeah, I knew that was probably wrong. But in any case, you can still use a vector.

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