# Movement of a helium filled vs lower-density-gas filled balloon inside an accelerated car

It's well known that a helium balloon inside of a car moves forward when the car accelerates, and backward when it slows down. What would happen, though, if a lower density gas was used instead of helium? Would the movement change? Would it stay the same?

One more question: How does the air pressure vary inside the car during acceleration?

• What are your thoughts on this so far? Commented Oct 24, 2018 at 0:25
• What do you mean here by the "deviation angle?" As far as I know, the balloon always moves in the direction of the acceleration. So when the car accelerates forward, the balloon moves forward within the car. If the car turns left (accelerates inward/to the left), the balloon should move that way too... I don't think there's any other complicated angle in play. And while the effect would be more pronounced the more different the balloon pressure is from the air pressure, there should otherwise be no change in this behavior. Commented Oct 24, 2018 at 3:21
• I should have said that the balloon is tied to the ground, and the deviation angle is the angle between the thread and the vertical line. I've added an image to illustrate. My way to solve it was to put it in the non-inertial reference of the car, where the gravity acceleration and the fictitious acceleration sum into a resulting acceleration that should have the same direction of the buouyancy force the air makes in the balloon. This way, considering that the balloon stabilizes in an angle, it should depend just on the acceleration of the gravity and of the car, but not on the gas density. Commented Oct 25, 2018 at 0:09