# Physics Simulation Forum

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 Post subject: Angular Velocity 2 questionsPosted: Mon Mar 21, 2011 2:46 pm

Joined: Sat Jul 31, 2010 10:30 am
Posts: 4
Hi All

I want to specify an initial angular velocity for some bodies before to start simulation. If I set a rotation around single axis, it works fine. But I can't figure out how it works for 2/3 axes. I have created cones along x, y, z and tried to find rotation order experimentally - but nothing does work. For example rotation angles (0, 1, 1 radians) do NOT produce same result as (0, 0, 1) for cone along Y. So

1) What rotation order (in Euler angles) should I specify for angular velocity?

2) How can I specify angular velocity in local space? For example there are 2 bodies: cube (parent) and cone (child). Parent cube is rotated and child cone inherits this rotation. How to applt cube's btTransform to cone's angles?

Thanks

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 Post subject: Re: Angular Velocity 2 questionsPosted: Thu Mar 31, 2011 9:21 am

Joined: Wed Mar 12, 2008 9:08 am
Posts: 23
In the real world, a unconstrained body not influenced by any forces can only rotate along one axis. Look at the Earth as an example. If Bullet allowed you to make an object rotate about more than one axis at a time, it wouldn't be physically correct.

Cheers,
Mike

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 Post subject: Re: Angular Velocity 2 questionsPosted: Sat Jan 14, 2012 4:26 pm

Joined: Thu Nov 17, 2011 6:32 pm
Posts: 8
In Blender, at least, I think there are options for using local coordinates rather than world ones. And, are you sure you're not confusing rotation (changing the object's orientation) with rotation velocity? The first responder was correct that an object can have only one axis of rotation. Why would you expect that, say, (1,0,0) would give you the same axis as (1,1,0) (unless you think you've got an object angled/tilted between two of the axes, and one is using local coordinates)?

In Blender, at least, one sets the angular velocity in the "logic editor" menu's motion actuator area.

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