# Helium balloon in a decelerating car

1. Nov 12, 2009

### sganesh88

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.

2. Nov 12, 2009

### ZapperZ

Staff Emeritus
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. Nov 12, 2009

### Ranger Mike

now that is neat!
thanks!!!!

4. Nov 12, 2009

### Staff: Mentor

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. Nov 12, 2009

### sganesh88

Sorry. I meant to type floor panel.