iPhone keeps elements in layer basis and you can handle any layer using CALayer class. You can rotate an object of a layer wrt other layer.
Here is a link that helps you to understand the layer geometry: https://developer.apple.com/iphone/library/documentation/Cocoa/Conceptual/CoreAnimation_guide/Articles/Layers.html
Every object has 2 points and 2 frames in a layer.
The points are: 1. position and 2. anchorPoint
1. position: position means the coordinate of rotation/weight point wrt system coordinate.
2. anchorPoint: The distance of rotation/weight point from x bound and y bound.
The frames are: 1. bounds and 2. frame
1. bounds: The coordinate of each object wrt to that object(0.0,0.0,width,height)
2. frame: The coordinate of each object wrt to system coordinate(x,y,width,height)
Formula to set rotation point:
if rotation point is (X,Y) and anchorPoint=(A,B)
X = x + width*A
Y = y + height*B
using this eqn. you can calculate any value that you require.