title: Swift adding relative force to a physics body
body: <p>I have a question.
  I am busy with a game (for educative purposes for myself) in Swift for OSX.
  I am trying to create a car which can be turned around and can be moved.
  I use this update function: </p>
  
  <pre><code>override func update(currentTime: CFTimeInterval) {
      self.moveMentForward()
      self.moveMentSteer()
  
      var point: CGPoint = CGPoint(x: self.car.size.width, y: self.car.size.height/2)
      var force2: CGVector = CGVector(dx: self.directionForwardForce!, dy: 0)
  
      self.car.physicsBody?.applyForce(force2, atPoint: point)
      self.car.physicsBody?.applyAngularImpulse(CGFloat(self.directionSteerForce!))
  }
  </code></pre>
  
  <p>But the applyForce is always in the X and Y direction, so the car will only move to the left or right.
  But i want the car to be moved to different directions, like up and down or in a different direction.
  So actually the force has to be applied always to the back of the car, like in this images:
  <a href="https://i.stack.imgur.com/QxXnL.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/QxXnL.png" alt="force 1"></a>
  
  <a href="https://i.stack.imgur.com/x4Z9P.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/x4Z9P.png" alt="force 2"></a>
  </p>
  
  <p>Is this possible with Swift and if so, how is this possible?
  Thank you very much already.</p>