javascript - Prevent object from leaving camera bounds -

i'm playing little phaser framework , try make simple "fall down" game. goal fall fast enough without getting pushed out @ top of screen:

to accomplish set canvas size 800x600:

var game = new phaser.game(800, 600, ...); 

and resizing world in create() 800x6000:

game.world.resize(800, 6000); 

in update() move camera 1 down:

game.camera.y += 1; 

and check if ball still inside camera:

if (!ball.incamera) {    // ...          } 

my question is:
on left , on right world borders limiting moveability of ball (because of ball can't leave camera there). how can prevent ball "falling" out camera on bottom, still able pushed out camera @ top?

is there similar to

game.physics.arcade.checkcollision.down = true; 

but camera bounds?

edit

this how ball created:

ball = game.add.sprite(game.world.width / 2, 20, "ball"); game.physics.arcade.enable(ball); ball.body.gravity.y = 1000; 

to move ball check key presses , change ball.body.velocity parameters.

physics has nothing camera (and indeed shouldn't), elegant solution create invisible body, align top bottom camera bound , move camera, still allowing ball moved out of view @ top, not allowing fall down.