title: how to change the speed of falling nodes in SWIFT
body: <p>I would like to know how to change the speed of falling nodes in sprite kit using swift, I have tried by changing the gravity, but when it goes very fast it starts to crash.
  I have done this, it works, but as I said it crashes:</p>
  
  <pre><code>var velocity:CGFloat = 0
  override func update(currentTime: CFTimeInterval) {
  velocity = CGFloat(score*3)
  self.physicsWorld.gravity = CGVectorMake(0, -velocity)
  }
  </code></pre>
  
  <p>Thank you!</p>