There are two different physics engines in Unity, one for 3D and one for 2D. While both use the same concepts, such as gravity, force, and velocity, they are implemented in slightly different ways, especially since there isn't a third dimension in 2D games. There are several components and properties when it comes to 2D physics in Unity. However, the two main important components to understand this book will be Colliders and Rigidbody, since we are going to use them both to implement our PlayerController.