Why does a grounded component move?

[lemelman]

I've got a simple model consisting of two arms linked together at one end with a revolute joint, and I've grounded the top arm. As expected, I can move the bottom (ungrounded) arm, but not the top one. If, however, I animate the joint, the top (grounded) arm moves and the ungrounded arm remains stationary. Am I missing something?

[jhackney1972]

Please refer to @jeff_strater post for a better answer.

[jeff_strater · Accepted solution]

Animate Joint only exercises the specific joint created, not the whole system.  So, all other joints are ignored.  This is the one place where the order in which you select the components when building a joint is significant.  Animate Joint shows the first component selected moving.  Try using Animate Joint Relationship (used to be called Animate Model).  It will obey all joints in the model.

[lemelman]

So, under certain circumstances, grounding a component doesn't actually ground it. I've accepted your solution but fail to see the logic.    It seems rather like a case of--

'When I use a word,' Humpty Dumpty said in rather a scornful tone, 'it means just what I choose it t...

Lewis Carroll

[davebYYPCU]

Grounding does work, including Animate Model,

and is only released with Animate Joint.  

Any other time it’s not working, should be reported.

Might help.....

[jeff_strater]

"So, under certain circumstances, grounding a component doesn't actually ground it"

No.  This is specific to this one command.  I agree that it could be confusing.  Animate Joint is exactly what it says:  It animates THAT joint, in isolation from the rest of the system.  This is the same thing that you see when you create the joint, and click the Preview Motion button in the dialog.  That preview, also, temporarily ignores all other joints in the system.  The ground is there and still active for any system-wide animation (Drive Joint, Animate Joint Relationships, component drag, etc)