# Helium Balloon in a Car - How Does It Move When You Decelerate **URL:** \ **Category:** Factual Questions\ **Created:** [September 24, 2004, 12:32am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786 "2004-09-24T00:32:21Z")\ **Posts on this page:** 10\ **Page:** 2
**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\ **Post date:** [September 24, 2004, 4:49am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/21 "2004-09-24T04:49:18Z")
> [@scr4](#): > > Which is why the _air_ rushes towards the front of the car when you brake. The balloon also wants to do the same, but is pushed aside by air because a helium balloon is lighter than air. Then if air can block the forward motion of a helium-filled balloon, in a sealed car with no air it would act the opposite way, right? If the air pressure builds up so much when you brake, why aren’t your ears popping at every stop sign? An ear is a very sensitive pressure measuring device. ---
**Author:** ![Berkut](https://avatars.discourse-cdn.com/v4/letter/b/a88e57/32.png) [@Berkut](https://boards.straightdope.com/u/Berkut)\ **Post date:** [September 24, 2004, 4:57am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/22 "2004-09-24T04:57:38Z")
Not sensitive enough in this case. Do your ears pop when you climb a stepladder? It’s a very small difference in pressure. Enough to affect the balloon, though. ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [September 24, 2004, 5:14am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/23 "2004-09-24T05:14:38Z")
> [@Musicat](#): > > TIf the air pressure builds up so much when you brake, why aren’t your ears popping at every stop sign? An ear is a very sensitive pressure measuring device. Think about how a balloon floats in the first place. Gravity is pulling it down. Gravity also pulls on the surrounding air. Because air is heavier than a helium balloon, it presses down harder, displacing the balloon. When you brake, the air, in addition to pressing down on the car floor, pushes slightly towards the front of the car. The ballon is displaced towards the rear of the car. The actual change in pressure is very tiny - difference - as Berkut said, comparable to the pressure difference you get by climing a steplader. (Or more likely, just a single step on that steplader.) ---
**Author:** ![W.Fikere\_Tomba](https://avatars.discourse-cdn.com/v4/letter/w/f9ae1b/32.png) [@W.Fikere\_Tomba](https://boards.straightdope.com/u/W.Fikere_Tomba)\ **Post date:** [September 24, 2004, 5:24am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/24 "2004-09-24T05:24:22Z")
> [@Musicat](#): > > Then if air can block the forward motion of a helium-filled balloon, in a sealed car with no air it would act the opposite way, right? I would imagine so (assuming you have a reinforced balloon, so it doesn’t explode). The balloon would also not float in a vacuum. I really like scr4’s explanation. Gravity is just a form of acceleration like the car’s change in speed. So in an airless car, the balloon would sink to the floor, and when you braked, it would move forward. Just as the presence of air causes the balloon to rise, it also causes it to move backward when you brake. ---
**Author:** ![matt](https://avatars.discourse-cdn.com/v4/letter/m/a6a055/32.png) [@matt](https://boards.straightdope.com/u/matt)\ **Post date:** [September 24, 2004, 6:45am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/25 "2004-09-24T06:45:01Z")
If you have no car, you can model the situation with a wine bottle, two cork stoppers, some cotton thread and some water. First, get two wine bottles with cork stoppers. At least one of them must be a cylinder type of stopper, as opposed to the mushroom type. Then, drink the wine. If you drink all of it on your own in one session, wait for your motor skills to return. Or lose interest, forget what you were doing and throw the corks away. Now you’ll need to buy some more! Tie a cotton thread to one of the corks. The best way is to actually pin or staple it to one end, because you are going to shove that cork back through the neck into the bottle. Alternatively, split the cork lengthways and use a half cork - then you can tie the thread around it. Shove the cork or half-cork back through the neck into the empty bottle. Pay out the thread until the cork is dangling about two thirds the way down the bottle. Clamp the thread to the outside neck of the bottle with your fingers. Fill the bottle with water, to the very top. The cork inside will float, but shouldn’t come out of the neck unless you’re using a half-cork and you’re very unlucky. Shove the other cork into the bottle’s neck, pinning the thread and sealing the bottle. Carefully, turn the bottle upside down so the tethered cork is now floating in the water, string vertical. Start moving the bottle around. Marvel as the cork moves in the same direction as you shove the bottle, in defiance of all logic! Ain’t physics great? My school physics teacher showed me this one, (using a cork and a round flask stopped with a rubber bung) and asked me to predict what would happen. I got it wrong. ---
**Author:** ![tomndebb](https://avatars.discourse-cdn.com/v4/letter/t/b9e5f3/32.png) [@tomndebb](https://boards.straightdope.com/u/tomndebb)\ **Post date:** [September 24, 2004, 6:59am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/26 "2004-09-24T06:59:59Z")
> [@](#): > > If the air pressure builds up so much when you brake, why aren’t your ears popping at every stop sign? An ear is a very sensitive pressure measuring device. As noted, there is not sufficient pressure to affect the eardrum (tht is made of fairly tough stuff. It is possible to feel the air moving inside (some) cars when one brakes. I used to drive a Dodge Caravan with a four-banger and an anemic heater. In the winter, on tight turns or hard brakes, I could feel cool air swirl around my legs as the cooler air in the back was swept forward, pushing the accumulated warm air out of the way. In the summer I would get a similar effect (with the temperatures reversed) as the air in the back of the car overwhelmed the air conditioner’s output. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [September 24, 2004, 10:19am UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/27 "2004-09-24T10:19:46Z")
Gravity is acceleration; (acceleration is not gravity, but for the purposes of this experiment it behaves as if it is); when the car accelerates, the objects in the car experience ‘g forces’ pressing them backwards the balloon ‘floats’ (in exactly the same wayand for the same reasons that it ‘floats’ upwards normally) toward the front of the car because the denser air surrounding ‘sinks’ toward the back. On deceleration, the same thing happens in the opposite direction, for the same reason. ---
**Author:** ![Ethilrist](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ethilrist/32/4604_2.png) [@Ethilrist](https://boards.straightdope.com/u/Ethilrist)\ **Post date:** [September 24, 2004, 2:30pm UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/28 "2004-09-24T14:30:51Z")
> [@W. Fikere Tomba](#): > > Do only less-dense-than-air objects behave this way, or would the pressure difference affect any light objects in the car? If you hung a sheet of paper from the ceiling on a string, would it also move back when you decelerated? > > I _would_ try this myself, but I don’t have a car. Well, use a bicycle, then. wait a minute… ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [September 24, 2004, 3:43pm UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/29 "2004-09-24T15:43:22Z")
The balloon moves forward if the string is attached to the floorboard and the balloon is suspended in midair. When the car starts forward there is a pressure wave in the air inside generated by the back of the passenger compartment that starts the air in the car moving with the car. This pressure wave travels forward through the car accelerating each incremental volume of air forward. When the wave comes to the ballooon, the density of the balloon is less than air and so it is accelerated more than the surrounding air and so it moves forward. As another poster said, try it. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [September 24, 2004, 5:16pm UTC](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786/30 "2004-09-24T17:16:31Z")
My post dealt with acceleration forward. In deceleration the front of the car produces a pressure wave that travels backwards in the cabin and the balloon goes backwards. [Previous page](https://boards.straightdope.com/t/helium-balloon-in-a-car-how-does-it-move-when-you-decelerate/265786.md?page=1)