title: SKPhysicsJoint allow orbit around a fixed point
body: <p>Using spritekit physics joints how could you make an object orbit another at a fixed distance?</p>
  
  <p>Object-A, should not move
  Object-B, should orbit Object-A when a force is applied to it</p>
  
  <p>To achieve this I tried attaching a SKPhysicsJointLimit limit between the two physics bodies and then apply a force to Object-B.  I consider the behaviour that results to be odd, but it may not be... What results is that Object-B jitters about but continues off for ever and doesn't seem to be constrained by Object-A which has not moved at all. </p>