var game = new Phaser.Game(800, 600, Phaser.AUTO, 'phaser-example', { preload: preload, create: create });

function preload() {

    game.load.image('ball', 'assets/sprites/yellow_ball.png');

}

var ball;

function create() {

    ball = game.add.sprite(300, 0, 'ball');

    startBounceTween();
}

function startBounceTween() {

    ball.y = 0;

    var bounce=game.add.tween(ball);

    bounce.to({ y: game.world.height-ball.height }, 1000 + Math.random() * 3000, Phaser.Easing.Bounce.In);
    bounce.onComplete.add(startBounceTween, this);
    bounce.start();

}