title: Weird behaviour from SKPhysicsBody applyImpulse
body: <p>So i have a few balls dropping from the sky, touching a paddle.</p>
  
  <p>The paddle applies</p>
  
  <pre><code>ball.physicsBody?.applyImpulse(CGVectorMake(0, 200))
  </code></pre>
  
  <p>to the balls.</p>
  
  <p>9 out of 10 times, the ball bounces back up 1/4 of the screen.</p>
  
  <p>1 out of 10 times, the ball goes super crazy and flies as if applyImpulse would've been (0, 1000)~</p>
  
  <p>Also!
  When the balls return back from the first bounce, and get .applyImpulse once again, they fly all the way to the top. Much much higher than the first applyImpulse. (Happens on all balls, first bounce: 1/4 screen. Second bounce: 3.9/4 screen.)</p>
  
  <p>Why is this?</p>
  
  <p>I would like the balls to stay consistent, i want every bounce to have the same force. </p>
  
  <pre><code>    ball.physicsBody = SKPhysicsBody(rectangleOfSize: CGSize(width: ball.size.width, height: ball.size.height))
      ball.physicsBody?.categoryBitMask = PhysicsCategory.ball.rawValue
      ball.physicsBody?.contactTestBitMask = PhysicsCategory.bar.rawValue | PhysicsCategory.boss.rawValue
      ball.physicsBody?.collisionBitMask = PhysicsCategory.noCollision.rawValue
      ball.physicsBody?.affectedByGravity = true
      ball.physicsBody?.dynamic = true
  
  func didBeginContact(contact: SKPhysicsContact) {
      let firstBody : SKPhysicsBody = (contact.bodyA.categoryBitMask &lt;= contact.bodyB.categoryBitMask) ? contact.bodyA : contact.bodyB
      let secondBody : SKPhysicsBody = (contact.bodyA.categoryBitMask &gt;= contact.bodyB.categoryBitMask) ? contact.bodyA : contact.bodyB
  
      if (firstBody.categoryBitMask &amp; PhysicsCategory.bar.rawValue ==  PhysicsCategory.bar.rawValue &amp;&amp;
          secondBody.categoryBitMask &amp; PhysicsCategory.ball.rawValue == PhysicsCategory.ball.rawValue) {
              CollisionWithBar(firstBody.node as! SKSpriteNode, ball: secondBody.node as! SKSpriteNode)
  
      }
  
  }
  
  func CollisionWithBar(bar: SKSpriteNode, ball: SKSpriteNode) {
      ball.physicsBody?.applyImpulse(CGVectorMake(0, 200))
      NSLog("Ball hit Bar")
  
  }
  </code></pre>