About Locking Rigid Bodies
You can lock rigid bodies relative to a lead body over a specific time period during an animation. The locked rigid bodies will act as if they are glued together, with no movement between them.
A rigid body lock does not take effect until a defined start time. If you drag or move your mechanism into a different configuration before the rigid body lock takes effect, you may have unexpected results.
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Rigid body positions at each key frame are specified by a snapshot. A locked rigid body can override the key frame position if it has been defined as Unspecified or Desired in the key frame sequence. If the rigid body lock ends before the key frame sequence, rigid body position is determined by the new snapshot. As a result, rigid bodies may jump to a new position, as if they were defined as Desired.
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