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# Physics question

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Hi, I''m now busy writing the physics in my engine. I have a type that contains all the info for a physics object: model location forces on all 3 axes moments on all 3 axes speed for all 3 axes rotational speed for all 3 axes my question: Should the rotational speed be in an object coordinate system instead of the worlds coordinate system? I used my lighter (use whatever object, this one was the nearest i could find and throw with) to try this, but it''s hard to really get to know how it works. This is the setup of my little experiment: - Give a big swing to the lighter so it rotates around the z axis with a great speed. - Swing it softly again around de x axis. Now my lighter should keep turning fast around it''s own z axis and keep turning slow around it''s own x axis, no matter what orientation in the world, right? Hope it''s clear what I mean, this is hard to explain. Marty.

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The rotational speeds are scalar though right? They apply to whatever axes you want them to. It sounds to me like what you want is the model''s coordinate system, not the world''s.

What about frictional coefficient? Elastic coefficient?

Regards,
Jeff

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the rotational speeds are indeed scalars, for every axe one, so actually it's a vector projected onto all the axes. That also goes for the (directional) speed, moments, forces...
What I don't want to rotate around the objects axes, but:

To know if the rotational speed of an object should be in the worlds system of axes or in its own system of axes.

Then I'll choose how to rotate

Marty

[edited by - Marty666 on March 31, 2003 2:07:20 PM]

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You should be thinking here in terms of torque. Initially, you angular momentum is along the Z axis (world). By giving it a "push" to make is swing around the X axis (ie, applying an impulsive torque) you are "pushing" the axis of rotation (angular momentum) from the Z axis (world) towards the X axis (world). So you result is a rotation axis somewhere between Z and X. You then need to accumulate these rotations in a matrix (or quarternions to avoid accmulating error) to get you object matrix.

Thinking about rotations can hurt your head - you can''t simply break it into components and do "a little about x" followed by "a little about z" or anything like that.

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Thanx, this is what I meant with the rotations being in de objects coordinate system. I''ll use that.
I was planning to use quaternions, so I''ve been studying them a bit. I get the methods used and how it works (I have a subject on linear algebra on the university), but what does it have to do with complex numers?

Marty

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