In lesson 1 of the physics tutorial when transitioning between the ground box to creating the box that falls there is a small thing which may confuse some for a moment.
Here it talks about the ground:
The box by default has height, width and depth of 1, we resize it to be 10x10 thin (0.5) floor.
floorBox.getLocalScale().set( 10, 0.5f, 10 );
the next sentence it talks about the box which drops but notice how it says we want the box to fall down to the floor. This makes it sound like it is referring to the box above, but it is not it is referring to he box which will be created a few lines later.
We want the box to fall down to the floor. To do this we create a DynamicPhysicsNode. A DynamicPhysicsNode can be moved, unlike a StaticPhysicsNode. It's also automatically affected by a downward gravity force. Again, no direct instantiation.