The Panda 1.7.0 distribution comes with a Python exposure for PhysX, which is called Panda PhysX. Panda PhysX is a Python library you will use to access the Physx API from a Panda script.

1. To begin with, you need to import the Panda PhysX classes you will use along the code.

   from panda3d.physx import PhysxManager
   from panda3d.physx import PhysxEnums
   from panda3d.physx import PhysxSceneDesc
   from panda3d.physx import PhysxBodyDesc
   from panda3d.physx import PhysxActorDesc
   from panda3d.physx import PhysxBoxShapeDesc
   from panda3d.physx import PhysxPlaneShapeDesc
   from panda3d.physx import PhysxPointOnLineJointDesc

1. The PhysxManager is your entry point to the PhysX engine. It is a singleton, that is a class that can have only a single global instance.

   self.physx = PhysxManager.getGlobalPtr()

   The physics simulation take place into a PhysX scene, a sort of side invisible world where the objects have their physical incarnation. So, to begin with you want to create a PhysX Scene. Every time you want to create a general PhysX object, you need to provide a descriptor first and pass it as a parameter to a specific creation method. A descriptor is a blueprint of the object that you want to build and the creation method will return an instance of it. This fancy-looking way of instantiating classes is actually a variant of the Factory Method Design Pattern and it has several benefits.

   # Setup the physical world
   sceneDesc = PhysxSceneDesc()
   sceneDesc.setGravity(Vec3(0, 0, -9.81))
   self.scene = self.physx.createScene(sceneDesc)
   # Material of the world
   m0 = self.scene.getMaterial(0)
   m0.setRestitution(0.1)
   m0.setStaticFriction(0.3)
   m0.setDynamicFriction(0.3)

   For each object you want to be part of the physics simulation, you have to create a PhysX Actor and set it up with a PhysX Shape (for collisions) and a PhysX Body (for rigid body dynamics). You can set both shape and body in the actor descriptor and then create the actor into the physics scene. Here is how a box actor is created (note that you still need your original Panda model of the box, that's the visible version of the box):

   # Collision shape for the box
   shapeDesc = PhysxBoxShapeDesc()
   shapeDesc.setDimensions(Vec3(0.3, 0.3, 0.3))
   # Rigid body for the box
   bodyDesc = PhysxBodyDesc()
   bodyDesc.setMass(10.0)
   # PhysX actor for the box
   actorDesc = PhysxActorDesc()
   actorDesc.setBody(bodyDesc)
   actorDesc.setName('Box')
   actorDesc.addShape(shapeDesc)
   actorDesc.setGlobalPos(Point3(1, 0, 2))
   self.boxActor = self.scene.createActor(actorDesc)

   To see the effect of gravity on the box, you will now have to start a task that copies the position of the physical box to the position of the visual box at every frame, and advances the physics simulation world by one step:

   #--- PhysX ----       
   taskMgr.add(self.simulate, 'PhysX Simulation')
    
   def simulate(self, task):
       dt = globalClock.getDt()
       self.scene.simulate(dt)   # Split the computation from...
       self.scene.fetchResults() # ...results fetching so to support multi-threading
       # Set the position and rotation of the graphical box from the physical box
       self.box.setPosQuat(self.boxActor.getGlobalPos(), self.boxActor.getGlobalQuat())   
       return task.cont

   Test this and make sure gravity is working as expected. Make sure to comment out the LerpPosInterval for the box since you no longer need it! You should also change the left mouse button handler so that it sets the position of both the visual box and the physical box back to the original position (to maintain similar functionality as before). You will also want to set the linear velocity of the box to 0 like this self.boxActor.setLinearVelocity(Vec3(0.0, 0.0, 0.0)), otherwise the box will keep going faster and faster.

1. To introduce collision with the door, you will need to create an actor for the door with both collision shape and rigid body, just like you did for the box. Try using dimensions of 1.0, 1.0, 0.1 and mass of 500.0. Don't forget to update the door's visual position based on the physical position in the simulation task! Your simulate task should now look like this:

   def simulate(self, task):
       dt = globalClock.getDt()
       self.scene.simulate(dt)
       self.scene.fetchResults()
       self.box.setPosQuat(self.boxActor.getGlobalPos(), self.boxActor.getGlobalQuat())
       self.door.setPosQuat(self.doorActor.getGlobalPos(), self.doorActor.getGlobalQuat())
       return task.cont