code: function(rigidBody){
          if(rigidBody.parent){
              throw new Error("It is not possible to be added to more than one world one of the rigid body");
          }
          rigidBody.parent=this;
          rigidBody.awake();
          for(var shape=rigidBody.shapes; shape!==null; shape=shape.next){
              this.addShape(shape);
          }
          if(this.rigidBodies!==null)(this.rigidBodies.prev=rigidBody).next=this.rigidBodies;
          this.rigidBodies = rigidBody;
          this.numRigidBodies++;
      }
docs: I'll add a rigid body to the world.
  Rigid body that has been added will be the operands of each step.
  @param  rigidBody  Rigid body that you want to add