anchor: Body:getLinearVelocity
  Gets the linear velocity of the Body from its center of mass.
  
  The linear velocity is the ''rate of change of position over time''.
  
  If you need the ''rate of change of angle over time'', use Body:getAngularVelocity.
  
  If you need to get the linear velocity of a point different from the center of mass:
  
  * Body:getLinearVelocityFromLocalPoint allows you to specify the point in local coordinates.
  
  * Body:getLinearVelocityFromWorldPoint allows you to specify the point in world coordinates.
  
  See page 136 of 'Essential Mathematics for Games and Interactive Applications' for definitions of local and world coordinates.
  x, y = Body:getLinearVelocity()
  x number The x-component of the velocity vector
  y number The y-component of the velocity vector