Как сделать так, чтобы объект перемещался на экране Screen Manager

from kivy.app import App
from kivy.uix.widget import Widget
from kivy.uix.button import Button
from kivy.properties import (
    NumericProperty, ReferenceListProperty, ObjectProperty
)
from kivy.uix.screenmanager import Screen, ScreenManager, FadeTransition
from kivy.vector import Vector
from kivy.clock import Clock
from random import randint
from kivy.lang import Builder
 
Builder.load_file('menu.kv')
 
class PongBall(Widget):
    velocity_x = NumericProperty(0)
    velocity_y = NumericProperty(0)
 
    velocity = ReferenceListProperty(velocity_x, velocity_y)
 
    def move(self):
        self.pos = Vector(*self.velocity) + self.pos
 
 
class MainScreen(Widget):
    ball = ObjectProperty(None)
 
    def serve_ball(self, vel=(4, 0)):
        self.ball.center = self.center
        self.ball.velocity = Vector(vel[0], vel[1]).rotate(randint(0, 360))
 
    def update(self, dt):
        self.ball.move()
 
        if self.ball.y < 0 or self.ball.top > self.height:
            self.ball.velocity_y *= -1
        if self.ball.x < 0 or self.ball.right > self.width:
            self.ball.velocity_x *= -1
 
class GameScreen(Screen):
    pass
 
class MenuScreen(Screen):
    def start_game(self, event):
        self.manager.current = 'game'
        game_loop = MainScreen()
        game_loop.serve_ball()
        Clock.schedule_interval(game_loop.update, 1.0 / 60)
        return game_loop
        
sm = ScreenManager(transition=FadeTransition())
sm.add_widget(MenuScreen(name='menu'))
sm.add_widget(GameScreen(name='game'))
#sm.add_widget(PauseScreen(name='pause'))
 
class MenuApp(App):
    def build(self):
        return sm
 
if __name__ == '__main__':
    MenuApp().run()

<PongBall>
    size: 50, 50
    canvas:
        Ellipse:
            pos: self.pos
            size: self.size
            
<GameScreen>
    MainScreen:
                    
<MainScreen>
    
    ball: pong_ball
    
    canvas:
        Rectangle:
            pos: self.center_x - 5, 0
            size: 10, self.height
    Label:
        font_size: 70
        center_x: root.width / 4
        top: root.top - 50
        text: "0"
 
    Label:
        font_size: 70
        center_x: root.width * 3 / 4
        top: root.top - 50
        text: "0"
    PongBall:
        id: pong_ball
        center: self.parent.center
<MenuScreen>:
    canvas:
        Color:
            rgba: 0.8, 0.8, 0.8, 0.8
        Rectangle:
            size: self.size
            pos: self.pos
    BoxLayout:
        orientation: "vertical"
        padding: 50
        spacing: 5
        Button:
            text: "Start"
            font_size: 70
            on_press: root.start_game()
        Button:
            font_size: 70
            text: "Settings"
        Button:
            font_size: 70
            text: "Quit"
            on_press: quit()