Newbie questions aboout mixing physics and non-physics nodes

I'm new to 3D and jME and working through the tutorials.

If you're going to use jME physics, which Spatial nodes need to be created & managed by the PhysicsSpace?  All of them?

I had done the box+floor tutorial from the jME physics docs and I noticed that, when the game starts, the box falls to the floor… and sort of rocks back and forth when it lands.

I removed the floor and the box just fell into space, seemingly w/o means to stop.

So does that mean I have to create my terrain w/in the PhysicsSpace?  If I don't, will my physics-managed objects just fall through it?

What other sorts of objects must or should be in the physics space:

  - walls/trees/monumental structures (things you can't walk through)

  - shrubs/grass (things you can walk through)

  - mobs

  - chests/levers/doors (things you interact with)

  - just everything?

Simply any objects which you want to collide with each other need to be represented by physics objects.