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# Finding Precession

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I have a project I''m working on to try and get myself into the physics part of physics engine programming. Pretty much, I want to be able to figure out a final force based on the very basic forces applied to an object (for now, I''m restricting myself to rectalinear forces and torque as my basic forces). Now, I''ve managed to get a rectalinear system working based on Chris Hecker''s articles, and I understand the theory behind the rotational physics. The specific problem I posed for myself: Have a spinning cylinder (essentially a gyro) on an arm attached to a pole on which it is mounted. The pole is larger in height than the radius of the gyro. Through theory we know that the gyro will precess, but the experiment is set up in such a way that the gyro cannot precess due to restrictions on axis. What ends up happening, from equations in books, is the arm ends up spinning around the pole (which I believe is called precessional frequency). This problem as applied to my goal: I have gravity and a single torque acting on my spinning cylinder. If I run the sim with those conditions, the cylinder will end up spinning while falling down. What I''m wondering is how do I restrict the cylinder from moving downwards, yet still applying gravity to it, in essense forcing precession to occur. Once I figure that out, I don''t think finding the value of preccesion itself is going to be that problematic as soon as I figure out the restricting part. Any ideas? Thanks in advance -Evgeny

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Can you post a diagram of the situation you''re trying to model? I''ve read your post a couple times now, and I''m having trouble visualizing the problem you''re describing. In particular what degrees of freedom are involved in connecting the various bodies?

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Take a look at the following diagrams I quickly threw together please.

http://s93050300.onlinehome.us/evgeny/top.jpg
http://s93050300.onlinehome.us/evgeny/side.jpg
http://s93050300.onlinehome.us/evgeny/front.jpg

Delta L here is the precession. When you convert your angular momentum to angular velocity you end up with precessional frequency, which is in essence what I''m looking for.
The gyro is rotating around the arm, and is restricted to only rotation on the arm. The arm itself is free to rotate about the post (which it ends up doing).
Hope this helps, and thanks in advance,
-Evgeny

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I''m probably missing something, but how is the gyro attached to the arm? It looks like it should just fall right off, with or without rotation.

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For the time being, let''s say the gyro is attached on a frictionless bearing (I simply didn''t want to add the element of having friction involved) - and the arm being a cylinder, again attached to a pole with a frictionless bearing.
I used the object with points as tips for simplicity''s sake (plus, that''s how they had it in the book )
Thanks again,
-Evgeny

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What kind of bearing? Different kinds of bearings will give the gyro different degrees of freedom. At the very least, gravity will be counteracted by a force from the arm onto the gyro through the bearing. The exact type of bearing will determine the position of this force relative to the center of mass of the gyro.

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well, here''s a sample of a calculating version I put together (uses ideal equations, so this is in theory what the sim should look like):

http://s93050300.onlinehome.us/evgeny/nehegl.zip

by the way, the equation I used is precessional frequency = torque/angular momentum = Mgh / Iw

Hopefully this clears things up...
Thanks for patience,
-Evgeny

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