A floating helium balloon is in the backseat of a car. If the car accelerates forward, which direction does the balloon float and why?

When your car accelerates forward, the air inside moves back relative to the car. This creates a slightly high pressure in the rear of the vehicle and a low pressure up front. This can be visualized by thinking of a cup with water, where the water is replaced with air.

Since helium is lighter than air, it moves away from the region of high pressure. A similar balloon filled with CO_2 would move back, since it is heavier than the surrounding air.

Extra Resources
Check out this article about the Equivalence principle to learn more about the physics behind this balloon problem.

Here’s a great video to show you this question in action.