Helium balloon in a decelerating car

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    Balloon Car Helium
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Discussion Overview

The discussion revolves around the behavior of helium balloons in a decelerating car, exploring the physics behind their movement and the counterintuitive nature of their response during deceleration. The scope includes conceptual understanding and intuitive reasoning related to physics principles.

Discussion Character

  • Exploratory
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant describes a scenario where helium balloons in a decelerating car swing backward, questioning the underlying physics and suggesting that air movement creates a low-pressure region at the back of the car.
  • Another participant shares an anecdote from teaching, explaining that the behavior of the balloons is due to their lower density compared to air, which causes them to be displaced when the surrounding air is pushed back during deceleration.
  • A participant expresses curiosity and appreciation for the phenomenon, indicating interest in the topic.
  • Some participants seek clarification on whether the balloons in question are helium-filled or air-filled, noting confusion regarding the initial description of their suspension in the car.

Areas of Agreement / Disagreement

There is no consensus on the initial description of the balloons, as some participants seek clarification on their type and positioning. The discussion includes multiple viewpoints on the physics involved, indicating that the understanding of the phenomenon is still being explored.

Contextual Notes

Participants express uncertainty regarding the initial conditions and descriptions, which may affect their understanding of the scenario being discussed.

Who May Find This Useful

Individuals interested in physics concepts, particularly those related to fluid dynamics, buoyancy, and counterintuitive physical phenomena.

sganesh88
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I was watching a program "what would happen if" (or something along that line) in NatGeo and was stunned by what i saw in it. This is the situation.
Several helium balloons are suspended from a car's roof panel through strings (interior of the car). When the car decelerates, what direction do you think the balloons would swing to? forward? NO. That doesn't seem to be the way nature prefer. The balloons swung back in unison when the car was braked.
The anchor said something about the air in the car rushing front and as a result a low pressure region being created at the back.. Can someone clarify this further?
P.S: I think this thread could also be used to share other non-intuitive events.
 
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When I used to teach intro physics, I had my students get onto the Chicago CTA trains with helium balloons and see what happened when the trains accelerate.

The reason why these balloons float in the first place is that it is less dense than the air surrounding it. If you left it go, it will rise upwards because the more ends and heavier surrounding air sinks underneath the balloon and pushes it upwards.

Same thing when you accelerate. The air surrounding the balloon will get pushed back more, and thus, displaces the balloon. The opposite occurs when you decelerate.

BTW, this is one example where things happened counter-intuitively when one doesn't know the physics. It makes sense, and becomes intuitive when one learns a little bit more. So "intuition" in this case is nothing more than an accumulated body of knowledge.

Zz.
 
now that is neat!
thanks!
 
Clarification: is this a floating helium balloon or an air filled balloon hanging from the ceiling? The "suspended from the car's roof panel" part makes no sense to me.
 
russ_watters said:
Clarification: is this a floating helium balloon or an air filled balloon hanging from the ceiling? The "suspended from the car's roof panel" part makes no sense to me.
Sorry. I meant to type floor panel.
 

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