1.2g will do that.
Cars that can accelerate to 60 mph in the low 3-second range or faster will generally cause the occupants to experience in excess of 1g of lateral force. At that point, the force acting on you is greater than the force of the gravity pushing down, and you could, say, get your iPhone to be held to the seat as you accelerate. This is obviously what you're going to see in the following video, in this case with a Tesla Model S P85D being used as the vessel for the science experiment.
It's too bad that there aren't passenger seats in F1 cars, as watching this trick in one of those would be genuinely insane.