Panda3D offers four built-in choices to use for physics:
Panda’s Built-in Physics Engine: Panda3D has a very basic physics engine built-in that may apply forces to classes. The physics engine can handle angular or linear forces, as well as viscosity.
Bullet Physics Engine: New in 1.8.0. This is a good choice for more advanced physics in most games.
The Open Dynamics Engine: This is another option provided as part of the Panda3D binaries, but does not support as many features as Bullet.
When you have a very simple simulation, you will most likely want to use the built-in physics, which works with Panda’s collision system. Although, when the built-in engine doesn’t offer enough functionality for you, the Bullet system may be a better choice.