What do you mean by "iOS7 physics engine"? I can't imagine a game physics engine being included with the OS. Is this referring to a creative abuse of iOS UI widget animations?
SpriteKit - Contains a 2D physics engine similar to Box 2D but not as full featured (joints, rigid bodies, shapes, polygons). The whole engine is similar to Cocos2D but ships with iOS 7. A particle editor and texture atlas generator is now built-in to Xcode because of it.
UI Dynamics - Contains a set of constraints and joints which can be used to flip and throw views around the screen. Lots of API hooks for responding to / taking user interaction into account. This is used for effects like when you scroll the messages list (the message bubbles bounce and collide) or when you drop notification center down (it bounces).
Edit: from a brief look at the source code, this example uses the UIDynamicAnimator API, so it is simulated entirely with the UI dynamics stuff.
Same thing applies to UIKit Dynamics, except it plays along with UIKit stuff.
According to my tests it is not Box 2D, it produces significantly different results and only implements a subset of functionality. I've been told by others with more experience that it isn't Box 2D also, but if you have more information I'd like to know for sure.
https://ghostbin.com/paste/tm4ko
b2* shows up in there many times.
edit: slow pony