I want to create an invisible force field (a barrier); so I created a kinematic RigidBodyControl.
The problem is that I need a Geometry to place it (and move it) on the game world. I’ve tried with replacing it with a invisible Node but it doesn’t work. Removing the Material doesn’t work either (maybe exist some invisible Material?)
Directly create a RigidBody, without a spatial at all.
I’ve tried with this… no luck 
[java]
barrier_phy = new RigidBody(1.0f, new MotionState(){
@Override
public Transform getWorldTransform(Transform t) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
@Override
public void setWorldTransform(Transform t) {
throw new UnsupportedOperationException("Not supported yet."); //To change body of generated methods, choose Tools | Templates.
}
},new SphereCollisionShape(40.4f));
[/java]
Go on level higher,
take a look at the examples, there should be some using rigidbodys.
Also no reason to implement the motionstate yourself, the defaults should be fine.
http://hub.jmonkeyengine.org/javadoc/com/jme3/bullet/objects/PhysicsRigidBody.html
Thank you! 