Arcade body not collide with down world?



I want to allow my body go to the bottom with arcade body, but the others sides must collide with my body ball.
So i make this :

	var ball = this.physics.add.image(300,200, 'ball');
// i would all sides collide expect the down side

But it doesn’t works. On the doc however what i wrote is true :

Could you me help please ?