[austinz]

SpriteKit was released last year with iOS 7. It's a pretty much self-contained 2D game engine like Cocos2D, including stuff like physics simulation. iOS 8 is going to ship with SceneKit, which is similar but for simple 3D games.

[yowmamasita]

How extensive is the physics engine implementation?

[austinz]

Wikipedia claims that SpriteKit uses the Box2D engine internally: http://en.wikipedia.org/wiki/Box2D

[coldcode]

In iOS7 it is Box2d, but not all of it was implemented. There is likely a lot more now in iOS8. There is also a 3d version which I assume uses Box3d.