## Under acceleration, a helium-filled balloon inside of a car will jump forward in the direction of acceleration. I have been searching for the reason why with no definitive results. This is the dilemma: An acquaintance of mine, who has a degree from Johns Hopkins is attempting to argue for some magical force that drives the balloon forward, also that it has something to do with gravity.

Asked by: Eric from Sioux Falls, SD

The simplest way to think about the helium balloon in the accelerating car is to invoke Einstein's Principle of Equivalence: a constant acceleration is the same, in all respects, as a gravitational force. To make matters as simple as possible, we'll ignore the real force of gravity (i.e. what's pulling the car toward the Earth) and just think about the forward acceleration of the car.

Image Credit: Tessa Ricci

According to the Principle of Equivalence, the effect of this acceleration is exactly the same as a gravitational force pointing to the rear of the car. But everyone knows what a helium balloon, immersed in air, does in the presence of gravity: it rises, i.e. it moves in the direction opposite to the gravitational force.

So when the car accelerates, the balloon moves forward. Incidentally, the Principle also explains why, when the car accelerates, a passenger is jerked backwards: because, unlike the balloon, the passenger falls downward, in the same direction as the gravitational force.

In answering this question, I'm assuming you accept that a helium balloon rises in the presence of Earth's gravity. Because helium has a smaller density than air, the buoyant force of air will push it toward the sky against gravity. If you want a more detailed explanation of that, please submit another question.