Physics Simulation - iOS Game Programming - page 202

6. Now we have our surface ready to hold the physics world objects. Let us create a
new physics world object using the following line of code:
// 1
SKSpriteNode* circlularObject = [SKSpriteNode
spriteNodeWithImageNamed: @"ball.png"];
circlularObject.name = ballCategoryName;
circlularObject.position =
CGPointMake(self.frame.size.width/3,
self.frame.size.height/3);
[self addChild:circlularObject];