I had a friendly argument with my friend, and it turns out he's correct. I still don't know why I'm wrong though (haha).

Question: You are holding a bottle of soda in a car moving forward. The bubbles move upwards. Then the driver hits the breaks. Will the bubbles in the soda move forward or backwards with respect to the bottle initially?

Somehow, my answer is that the bubbles move forward with respect to the bottle, since if you just apply the breaks, the bubbles still don't feel the negative acceleration and still move upwards. Since the bottle does feel this acceleration, the bottle should accelerate in the negative direction, with the bubbles still moving upwards (and forward w.r.t the bottle).

Can someone explain why I'm wrong?


marked as duplicate by sammy gerbil, ZeroTheHero, Yashas, Qmechanic Apr 10 '17 at 3:35

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There is a great Smarter Everyday youtube video on this that has great explanations and footage of this same effect. There's also a wired article on the subject.

The balloons would move forward when the car decelerates if they were in a vacuum. The key is that the bubbles are sitting in a more dense fluid: the soda that they are in. The same thing happens with a helium-filled balloon in a car (that's otherwise full of air). Because the bubbles (helium balloon) are lighter than the surrounding cola (air in the car), they float---so if the heavier material tends to move forward with the deceleration, then the lighter material has to move in the opposite direction because it's displaced. The cola moves forward, so the bubbles have to move backward.


You can think of it like this. Bubbles move opposite to gravity, because they are lighter than the soda they're immersed in. When the driver hits the brakes, everyone feels a forward acceleration, so it's as if "gravity" suddenly had a forward component too. Therefore, the bubbles will move backwards.

Just to be clear: gravity still pulls down, not frontwards. But the bubbles don't care what force is acting, just that if the fluid is being pulled in some direction they will go opposite.


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