Re: [asa] Physics poser

From: <>
Date: Wed Sep 17 2008 - 23:13:27 EDT

Hi Jim,

My initial reaction:

Nobody would ask the question unless the answer was counter-intuitive!

therefore, (contra the obvious) the balloon would move forward.


Because air ALSO has mass and tends toward the rear of the vehicle under
acceleration. Being heavier than the helium filled balloon the air would
therefore move to the rear of the vehicle pushing the helium filled baloon

My "confirmatory" thought experiment is to imagine what would happen if the
car were half-filled with water - under acceleration water to the rear, air
forward. So lighter fluid displaced to the front of the vehicle when under

Thus endeth my speculations...

Murray Hogg

Quoting Jim Armstrong <>:

> Well, here's my favorite physics question:
> Imagine your car at rest at a stop light. A helium filled balloon lolls
> freely at rest at the center of the car's ceiling.
> Now imagine that the light turns green and you accelerate through the
> intersection.
> You notice that the balloon moves quite purposefully as you accelerate.
> Question: which direction did the balloon move, and why?
> Followup: set the experiment up for real and see what happens. Any
> surprises? Any explanation "adjustments" required?
> If you get it right, you'll have to settle for the satisfaction, I'm afraid!
> JimA [Friend of ASA]
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Received on Wed Sep 17 23:14:19 2008

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