Unity's 2D physics engine is very similar to its 3D one. Almost all of the physics components have been integrated into the 2D engine with a slight difference in names (Box Collider 2D, Circle Collider 2D, Rigidbody 2D, and so on...).
It is important to understand that 2D physics components will not interact with 3D physics components if both exist within the same scene. Even though they both share a lot of similarities, 2D physics only occur on the X and Y axis, and rotating an object using physics will only occur on the X axis.
If you wish to change any of the settings of the 2D physics engine, you can find all of its properties by navigating to Edit | Project Settings | Physics 2D.
Just like the 3D engine, the Rigidbody2D component controls the physical behavior of any game object it is attached to inside the scene. It also defines its physical properties, such as mass, and how gravity should affect an object.
To begin with rigid bodies, let's add this component to...