You have a balloon that is filled with helium. You tie it up in your minivan (or car, or truck, or whatever you happen to drive). You start to accelerate forward. Which way will the balloon travel?
It seems like it would be easy to intuit the answer. When a vehicle accelerates, the people and objects in the car are pushed backward. This means that, in the aforementioned scenario, the balloon should move toward the back of the minivan. Right?
Well, not quite.
The balloon will actually move forward because of air flow (similar to fluid dynamics). This may seem confusing or counter-intuitive, but the way that it works is actually quite simple.
The thing to remember is that the balloon floats because helium is less dense than air. So when the car accelerates, just like people are pressed into their seats, the air molecules that are in the minivan (or car, etc.) change their motion and travel toward the back of the vehicle. All of these molecules collecting in the back of the minivan have a clear impact on the balloon, forcing it forward.
In short, the air displaces the helium-filled balloon, making it move away from the rear of the car where all of the air molecules have collected. If this sounds confusing, you can watch the video by YouTuber Smarter Every Day to see this in action and get a clear explanation. In any case, be sure to try it the next time that you travel and explain it to your passengers (be they 9 or 90 years old).