Uia ! implementei meus rabiscos de gravidade e não eh q funcionou !! hehehe

como tinha dito em javafx-planejando-gravidade são 3 classes envolvidas:

• GravityManager – fazo update() a cada tick do jogo
• Force – aplica mudança de posição aos nodes
• GNode – associa Node a Force.

Para os catucadores de plantão segue o endereço do subversion
http://svn.vacavitoria.com/cabecudinhos_1/cabecudinhos_1/

E alguns fragmentos do codigo
esta eh a Scene em que usei a GravityManager

public class GameScene extends Scene, Updatable{
    ...
    public var gravityManager: GravityManager;

    // set the need info for the Scene work
    public function setup(){
        gameSoundtrack.play();

        background =
        lookup(BACKGROUND_ID) as NodeFromFXZ;
        player1 =
        lookup(PLAYER1_ID) as MovableChar;
        player1.background = background;
        player1.requestFocus();

        gravityManager = GravityManager{
            gravity: 5
            content: [
                GNode{
                    node: bind player1
                }
            ]
        }

    }

    public override function update(){
        gravityManager.update();
    }

    override var content = [
        NodeFromFXZ{
            id: bind BACKGROUND_ID
            source: "{__DIR__}media/background.fxz"
        }

        MovableChar{
            id : bind PLAYER1_ID
            source: "{__DIR__}media/boy.fxz"
            x: 50
            y: 450
            speed: 3
            colision: ["casa1", "casa2", "casa3","casa4", "casa5", "casa6", "casa7", "casa8",
             "caixa1", "caixa2", "caixa3","caixa4", "caixa5", "caixa6",
            "caixa7", "caixa7b", "caixa8", "caixa8b",
            "left", "right", "bottom","top", "plataforma1"]

            onMouseClicked: function(e){
            }

            onKeyReleasedDelegate: function(current:MovableChar, key: KeyCode){
               if( key == KeyCode.VK_SPACE and current.hasColision){
                   fire.play();
                   gravityManager.addForce( current.id, Force {
                        y: -22;
                   });
                }
            }

            onUpdate: function(current:MovableChar){
                current.checkIfScrollBackground();

            }
        }
    ]
}

Segue o codigo das classes para o controle da gravidade

GravityManager

public class GravityManager extends Updatable{

    public var gravity: Integer;
    public var content: GNode[];

    public function addForce(id:String, force: Force){
        for( gNode in content){
            if( gNode.node.id.equals(id) )  {
                insert force into gNode.forces;
            }
        }
    }

    public override function update(){

        // iterate over all gnodes
        for( gNode in content){
            var x: Integer = 0;
            var y: Integer = gravity;

            // iterate over all Force from the GNode
            for( force in gNode.forces){
                x = x + force.x as Integer;
                y = y + force.y as Integer;

                force.applyGravity(gravity);
                if( force.isDisposable()){
                    delete force from gNode.forces;
                }
            }

            // try to move the Node with the increase of x and y
            gNode.node.moveX(x);
            gNode.node.moveY(y);
        }
    }
}

Force.fx

public class Force {

    public var x: Number;
    public var y: Number;
    public var percentGravity: Number = 0.15;

    // reduce the x and y value with the gravity
    public function applyGravity(gravity:Integer){
        if( x < 0 ){
            x = x + (gravity * percentGravity);

            if( x > 0 ){
                x = 0;
            }
        }

        if( y < 0 ){
            y = y + (gravity * percentGravity);
            if( y > 0 ){
                y = 0;
            }
        }
    }

    public function isDisposable():Boolean{
        return x == 0 and y == 0;
    }

}

GNode

public class GNode {

    public var node: MovableChar;
    public var forces: Force[];

}

E finalmente o video com o resultado saltitante