Helium balloon in a decelerating car

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SUMMARY

The discussion centers on the behavior of helium balloons in a decelerating car, where the balloons swing backward instead of forward. This phenomenon occurs due to the air inside the car rushing toward the front, creating a low-pressure area at the back, which displaces the balloons. The explanation highlights the principles of buoyancy and air density, emphasizing that the less dense helium allows the balloons to rise and react counterintuitively during deceleration. Understanding these concepts requires a grasp of basic physics principles.

PREREQUISITES
  • Basic principles of buoyancy
  • Understanding of air density and pressure
  • Knowledge of Newton's laws of motion
  • Familiarity with the concept of acceleration and deceleration
NEXT STEPS
  • Research the principles of buoyancy in fluids
  • Study the effects of pressure differentials on object movement
  • Learn about Newton's laws of motion in practical scenarios
  • Explore real-world applications of physics in transportation systems
USEFUL FOR

Physics students, educators, and anyone interested in understanding the counterintuitive behaviors of objects in motion, particularly in relation to buoyancy and air pressure dynamics.

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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