anchor: Body:getLinearVelocityFromWorldPoint
  Get the linear velocity of a point on the body.
  
  The linear velocity for a point on the body is the velocity of the body center of mass plus the velocity at that point from the body spinning.
  
  The point on the body must given in world coordinates. Use Body:getLinearVelocityFromLocalPoint to specify this with local coordinates.
  vx, vy = Body:getLinearVelocityFromWorldPoint( x, y )
  x number The x position to measure velocity.
  y number The y position to measure velocity.
  vx number The x component of velocity at point (x,y).
  vy number The y component of velocity at point (x,y).