School-Safe Puzzle Games

Thought Experiment: Helium Baloon

Businessman with yellow balloon flying over labyrinth

If a truck is traveling with a helium-filled balloon inside the cabin, and the truck suddenly stops, what happens to the balloon?

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20 Comments to “Thought Experiment: Helium Baloon”


  1. Falwan | Profile

    some questions:


    truck traveling at constant speed?!!


    small helium-filled balloon floating freely in cabin?!!


  2. kasabubu | Profile

    It floats to the back of the cabin as it is lighter and therefore has less kinetic energy than the surrounding air. With a VERY sensitive barometer you should be able measure the difference in air pressure between the front and the back of the cabin since the air molecules float to the front. Or, with an even MORE sensitive thermometer you might measure an increase in temperature in the front and a decrease in the rear as compressing is exotherm and decompressing is endotherm.


  3. gpraghuram | Profile

    I think the balloon goes back inside the cabin.


  4. TonyTKL | Profile

    The balloon will move to the back of the truck because helium is less dense than air. So when the truck stops and the air pressure in the front of the truck increases, the balloon will move the other way.


  5. hex | PUZZLE MASTER | Profile

    If the truck is traveling in a straight line with no vertical slope and at a constant speed, the helium balloon will be stuck at some point of the cabin ceiling so that its relative velocity with respect to the cabin is zero (ie at rest). Its absolute speed, however, is that of the truck. When the truck stops, the balloon will keep moving forward until it hits the front of the cabin (similar to a person’s head crashing into the dashboard in an accident).


  6. Denita TwoDragons | Profile

    The balloon will stop, because it’s traveling relative to the truck. At most it might drift very gently forward until it hits the back of the driver’s head, prompting her to turn around and yell at her kid that he better hold on to that darn thing or it’ll be evicted out the window.


    Either that or everything will explode in a flaming catclysm of apocalyptic proportions.


  7. Shawn | PUZZLE GRANDMASTER | Profile

    Wow, something I actually remember from freshman physics!


    1. the balloon is not attached to the truck; it is free floating and weightless.


    2. helium is less dense than air, which is why the balloon floats.


    3.when braking, the air (which DOES have weight) in the truck cabin has momentum and continues to move toward the front of the truck, just like the driver’s body does.


    4. the onslaught of air creates an area of high pressure in the front of the cabin, and lowers the pressure in the back.


    5. the balloon moves toward the low pressure in the back of the cabin.


  8. ahsergio | Profile

    the balloon stays where it is.
    i hate phisics and i wouldnt be able to explain why, i just remember that a highschool teacher of mine commented about this renault add (released here, in Brazil)
    http://www.youtube.com/watch?v=s1kK_GAV430
    he said that the bee wouldnt feel the car acceleration.
    thats all folks!


  9. Mashplum | PUZZLE MASTER | Profile

    An object in motion will remain in motion until acted on by a force. So, according to Newton, the balloon will continue to move forward. However, it seems to me that all of the air molecules would move forward too, creating slightly higher pressure toward the front which would act against the forward motion of the balloon (assuming there is air in the truck as well).


    If the truck was full of water with an air bubble at the top, the water would move forward, and the air backward. I suppose a helium balloon is to air as air is to water, so the balloon will actually move back.


    The balloon will move backward. Final answer, Regis.


  10. Yar | Profile

    Just as the driver lurches forward with his/her momentum when the truck suddenly stops, so too does all the air in the cabin, pushing the balloon towards the back.


  11. archraider | Profile

    Im not sure if I understand the nature of this question but here goes:


    Relative to the direction of the truck,speed of the truck,density of the material containing the helium and whether it has a string on the balloon, the balloon will continue travelling in that direction until it is stopped by a window, wall or otherwise. If the force of the helium trying to rise, is stronger than the balloons inertia then it will stay in place……..Soooooo it will roll along the cieling?


  12. pgzzz | Profile

    In theory the balloon will carry on towards the front of the cab due to its (albeit small) momentum (A). This is offset by the friction between itself and the top of the cab (B) – (I assume there is enough helium in the balloon so that it will be touching the top). So as long as A is larger than B then it will move forwards.


  13. jmart | Profile

    Moving air should pressurize the front of the cabin and push the balloon towards the back.


  14. pgzzz | Profile

    Additional :-


    The other consideration is that the mass of the balloon must be sufficient to push through the air iside the cabin. This is because the air already fills the cabin and therore does not move forwards as the truck slows down.


  15. Hendy | Profile

    It like the driver would be pressed forward into the window because of the second law. Something in motion stays in motion until a force acts upon it. Bounce! Then it would go where ever the direction of the bounce sends it. Was this supposed to be a trick question?


  16. abcbcdcdef | Profile

    ah.. high school physics~


    The balloon would supposedly move in the opposite direction direction of the truck; as the denser amtospheric air would move in the direction of the truck, pushing the baloon backwards…no?


  17. hex | PUZZLE MASTER | Profile

    hmmm…. I wonder why I missed the air factor.
    Supposing that the total weight of the helium balloon is less than that of an equivalent volume of air (as is usual the case), there are several factors involved:
    1- There will be a higher pressure at the front temporarily, pushing back the balloon
    2- The balloon will have a momentum pushing it forward


    I suppose the conclusion will be as follows:
    - if truck speed is very high, the stop is really abrupt, and the balloon is at the back, it will start moving forward as most of the air will be compressed towards the front
    - Otherwise, it will move towards the back


  18. RK | Founder | Profile

    Hey there Hendy, haven’t seen you around in awhile!


    This page has a nice explanation also:
    http://www3.amherst.edu/~physicsqanda/Helans.htm


  19. hex | PUZZLE MASTER | Profile

    I beg to differ with the page (http://www3.amherst.edu/~physicsqanda/Helans.htm) in the extreme case I described:
    If the deceleration is high enough, and the cabin is air-tight to a certain degree, then most of the air will accumulate at the front, leaving a thinner atmosphere at the back (the air density will not be constant), so essentially the balloon falls forward!!


    This could even be the main reason why helium balloons (at least the non stretchable ones) do not rise beyond a certain altitude. The atmosphere becomes thin and less dense than helium.


    A specialist input would be highly appreciated.


  20. Mashplum | PUZZLE MASTER | Profile

    My problem with the amherst page is “In a perfect vacuum, say on the surface of the moon, the balloon – like everything else – would fall down.” I say the balloon would explode in a vacuum.


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