Helium balloon in a decelerating car

In summary, in a NatGeo program, a car's interior is shown with several helium balloons suspended from the floor panel. When the car decelerates, the balloons swing back in unison due to the displacement of air surrounding them. This is because the less dense helium balloon is pushed up by the heavier air. This counter-intuitive event can be better understood by learning the physics behind it.
  • #1
sganesh88
286
0
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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  • #2
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.
 
  • #3
now that is neat!
thanks!
 
  • #4
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.
 
  • #5
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.
 

Related to Helium balloon in a decelerating car

1. What happens to a helium balloon in a decelerating car?

When a car decelerates, it slows down in speed. This means that the car is still moving, but at a slower rate. The helium balloon inside the car will also experience this deceleration, causing it to move towards the back of the car due to inertia.

2. Will the helium balloon rise or fall in a decelerating car?

The helium balloon will rise in a decelerating car. This is because as the car slows down, the air inside the car is still moving at a constant speed. This creates an upward force on the balloon, causing it to rise towards the ceiling of the car.

3. Why does a helium balloon behave differently in a decelerating car compared to a stationary car?

In a stationary car, the air inside the car is also stationary. This means that there is no upward force acting on the helium balloon. However, in a decelerating car, the air inside the car is still moving at a constant speed, creating an upward force on the balloon.

4. Will the helium balloon behave differently in a decelerating car compared to an accelerating car?

Yes, the helium balloon will behave differently in a decelerating car compared to an accelerating car. In an accelerating car, the air inside the car is moving forward, creating a backward force on the balloon. This causes it to move towards the front of the car.

5. How does the size of the helium balloon affect its behavior in a decelerating car?

The size of the helium balloon does not significantly affect its behavior in a decelerating car. The main factor that affects its behavior is the speed at which the car is decelerating. However, a larger helium balloon may experience a slightly stronger upward force due to its larger surface area.

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