A Baffling Balloon Behavior - Smarter Every Day 113
Why Physics Stack Exchange recommends it
Physics Stack Exchange answerers point people to it as a clear experimental demonstration of why lighter-than-fluid objects like balloons and bubbles move opposite to the way heavier things do during acceleration.
Recommended by 3 people across 3 different months, 2015 to 2021.
What it is
Destin Sandlin of Smarter Every Day shows a helium balloon in a car that moves forward when the car accelerates, the opposite of what a heavier object does. He explains the effect through buoyancy and the pressure gradient that acceleration sets up in the air, with footage and other fluid examples.
What people said
Recommends Asher · 2015-01 · Physics Stack Exchange · on “Will buoyant force change if the container is accelerated up or down?”
In addition to the answers below, I'll link this video which shows an experimental treatment of the matter in a horizontal fashion: [this video]
Recommends DilithiumMatrix · 2017-04 · Physics Stack Exchange · on “Bubbles In Soda”
There is a great Smarter Everyday youtube video [this video] on this that has great explanations and footage of this same effect. There's also a wired [LINK] 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 di
Recommends Marko Gulin · 2021-12 · Physics Stack Exchange · on “If you have a object inside a car which has equal amount of inertia as car, will it stay stationary at any level of acceleration without going back?”
Assuming the car is closed and the ballon is in the car, when car starts moving forward the air has some inertia and will start building up at the end of the car. The air density will bi larger at the back of the car due to the air build up, and the ballon density does not change. This will push the ballon forward. Here is a nice experiment: [this video]
Cites Marijn · 2016-02 · Physics Stack Exchange · on “When a rocket or car is accelerating is the air in it going backwards or not?”
I'm not sure but when a car is accelerating is the air in the car pushed backwards or not? If so why did Einstein say that there is no way of finding out (besides tidal effects) the difference between being on the surface of the Earth, and being in a spaceship in deep space accelerating at 1g. But if a rocket is accelerating and the air in it is moving like in the car pushed backwards than a balloon is going also backwards in this rocket. But on the surface of the earth it is not moving (backwards) because the air and the ballon is under influence of the same gravity. So in this experiment you can see the difference if you are in a rocket or on the surface of the earth. Probably I miss somet
Cites candied_orange · 2019-11 · Physics Stack Exchange · on “A bob hanging in an accelerating train moves backward. What is the force moving it backward?”
@mckong it's very important to know not only what reference frame is being used but what moment in time is being depicted. The bob isn't rising at all now because this is a snap shot of a moment when the bob has stopped moving up or down. This is counter intuitive because it means the train has been constantly accelerating for an unusually long time to give the bob time to settle down. Try this experiment on a real train and you'll see a fair bit of swinging unless you put a motion damper on the bob. Here's a video of a similar experiment [this video] .