Question by kcazyz · May 21, 2013 at 04:00 AM · physics rigidbody addforce parent child

Child Gameobjects are not fixed to the parent when using physics

I am trying to make a flight simulator-type game, but I am having problems with the child gameobjects of the rigidbody. At the top level I have the root gameobject which contains the model, the rigidbody, and the airplane script. Then, that game object has a bunch of empty gameobject children that I use to set the position to apply forces. Each one of those gameobjects has an airfoil script which applies it's own forces on the main rigidbody using AddForceAtPosition.

I have been having some problems, however, and after some debugging I found that the position of the empty gameobjects relative to the root are moving. The local position stays the same, but if I log the root transform.position - an empty gameobject position I have noticed that that fluctuates quite a lot. Shouldn't this stay constant?

Do you think this has anything to do with AddForceAtPosition?

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Fornoreason1000 · May 21, 2013 at 04:08 AM 0

yes they should be moving...

transform.position shouldn't be constant if its moving. if you move a parent EVERYCHILD moves with it...`LocalPosition` is the position offset from the parent while Position is global...that's why local position didn't change and transform.position did.

And if your parent is moving, then so will be your child objects. why they do that is for practical reasons. e.g a player is carrying a gun, you add force to the player.. you want the gun to move with the player.

to fix your problem here's a hierarchy chart:

   -RigiBody
   -ContainerForEmpties 
        -Empty(s)

kcazyz · May 21, 2013 at 04:55 AM 0

I know I expect them to be moving, but they move incorrectly relative to each other. What I said was the difference between the global positions isn't constant. I ended up lowing the timestep and that fixed the problem for the most part. It seems like because the empty gameobjects that had the airfoil code were taking more time to do calculations they were a step or two behind everything else. This is why when the velocity was large they didn't move exactly with the parent.

Fornoreason1000 · May 21, 2013 at 05:46 AM 0

Ok, Now I understand, sorry for that. I was confused since you said