title: SpriteKit: SKPhysicsBody slips away
body: <p>I have a SKSpriteNode (<strong>Bird</strong>) with a SKPhysicsBody (<strong>Circle</strong>). 
  The bird node is affected by gravity and falls down, until it reaches a y position of <strong>100px</strong>.  </p>
  
  <pre><code>override func didSimulatePhysics() {   
  
          if (bird.position.y &lt; 100) {
  
                 bird.position = CGPointMake(bird.position.x, 100)
           }
  }
  </code></pre>
  
  <p>The problem is, that the physicsBody is moving away from the Node.
  The physicsBody should stay centered on the Bird.
  As soon as I tap the screen the Bird gets an impulse and moves up. Immediately the physicsBody is back in place.</p>
  
  <p><img src="https://i.stack.imgur.com/kixej.gif" alt="enter image description here"></p>
  
  <p>This is the physicsBody code:</p>
  
  <pre><code>bird.physicsBody = SKPhysicsBody(circleOfRadius: bird.frame.width * 0.5)
  bird.physicsBody!.contactTestBitMask = PhysicsCategory.LeftBoard | PhysicsCategory.RightBoard |  PhysicsCategory.Border
  bird.physicsBody!.categoryBitMask = PhysicsCategory.Bird
  bird.physicsBody!.collisionBitMask = PhysicsCategory.None
  bird.physicsBody!.mass = 1
  
  bird.physicsBody!.affectedByGravity = false
  bird.physicsBody!.allowsRotation = false
  </code></pre>