Adding a gravity effect to user interface elements in an OS X app?

Question

EDIT . To make the question clearer, I will quote what Chris said in a comment, the question: "How can I do on OS X what UIKit Dynamics does on iOS?"

I want to add a gravity effect to my user interface elements in my OS X application, and I wonder if there are any best practices?

If I were developing an iOS application, I would switch to the dynamic features introduced in UIKitin iOS7, but I could not find anything like that in AppKit. I even tried using Sprite Kitan AppKit-based application for physical modeling , but I could not find a way to remove the annoying gray background in SKView, even when I set backgroundto SKColor.clearColor().

So, I wonder, is gravity really a challenge? Are there any best practices? I found some applications, such as Pixelmator, integrated some cool gravitational functions, so I guess this is possible? Can someone help me get out of here? Many thanks!

A little discussion: with the release of OS X 10.10, Apple is abandoning skeuomorphism, is there still a good design idea to add physical modeling to your application? How is the rope effect in the Pixelmator app?