# Thread: Physics_model, rigid_body and rigid_constraint

1. ## Physics_model, rigid_body and rigid_constraint

I'm trying to understand how physics_model and its children are to be used to describe a collection of rigid bodies and constraints. I wonder if anyone can help by answering the following questions.

1. Does a rigid_body's <dynamic> child element specify whether the rigid body is movable with respect to the world, or only whether it is movable with respect to some containing physics_model?

2. Suppose a physics_model "A" contains rigid bodies B and C, but specifies no rigid constraints. Then are B and C (implicitly) linked rigidly, or are they completely unconstrained with respect to each other?

3. Suppose a physics_model "A" contains rigid bodies B, C and D, and specifies a rigid_constraint between B and C. Is component D then unconstrained, linked rigidly to B, or linked rigidly to C?

4. Suppose a physics_model "A" contains rigid body B and physics model C, and C in turn contains rigid bodies D and E. C also contains a rigid_constraint linking D and E. How can A specify a rigid_constraint linking B with, say, the D component of C? (Note that the description of rigid_constraint specifies that ref_attachment and attachment elements refer to rigid_body elements within the same physics_model, suggesting that a rigid_constraint in A cannot refer directly to either D or E.)

Thanks.

2. ## Re: Physics_model, rigid_body and rigid_constraint

Originally Posted by apope
I'm trying to understand how physics_model and its children are to be used to describe a collection of rigid bodies and constraints. I wonder if anyone can help by answering the following questions.

1. Does a rigid_body's <dynamic> child element specify whether the rigid body is movable with respect to the world, or only whether it is movable with respect to some containing physics_model?
If <dynamic> is set to true, it will be a moveable rigid body. If <dynamic> is set to false, it means it is fixed/static/kinematic. This can be useful static collision geometry.
2. Suppose a physics_model "A" contains rigid bodies B and C, but specifies no rigid constraints. Then are B and C (implicitly) linked rigidly, or are they completely unconstrained with respect to each other?
Without constraints, they are not linked. Adding several bodies and/or constraint so a physics_model is useful for multiple duplicating/referencing a complicated group of rigid bodies or constraints. They don't need to be all connected by constraints. You can make an group of unconstraint rigidbodies. This can be useful for pre-creating an avalange or a prefractured body (falling apart once the simulation starts).

3. Suppose a physics_model "A" contains rigid bodies B, C and D, and specifies a rigid_constraint between B and C. Is component D then unconstrained, linked rigidly to B, or linked rigidly to C?
Same as before, if no constraints are specified, there is no implicit links.

4. Suppose a physics_model "A" contains rigid body B and physics model C, and C in turn contains rigid bodies D and E. C also contains a rigid_constraint linking D and E. How can A specify a rigid_constraint linking B with, say, the D component of C? (Note that the description of rigid_constraint specifies that ref_attachment and attachment elements refer to rigid_body elements within the same physics_model, suggesting that a rigid_constraint in A cannot refer directly to either D or E.)
I have to doublecheck if hierarchical definitions of physics_models fall within the scope of 'within the same physics_model'.

Thanks,
Erwin

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