annaandherdad
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Balloons, and Principle of Equivalence
This is really physics more than chemistry, but it's simple and I think very interesting. I put it in Miscellaneous because I wasn't sure where else
to put it.
Of course hydrogen or helium balloons rise to the ceiling. Try putting one in your car. Let it rest on the ceiling. Now step on the gas
(accelerate). Your change on the dashboard will slide back, but the balloon will move forward. From the standpoint of a stationary observer on the
sidewalk, the balloon will accelerate forward faster than the car itself. I find it a little unnerving to see it.
Likewise, when you hit the brakes, everything in the car lurches forward, except the balloon, which lurches toward the back.
Or go around a corner, say, turn right. All your change on the dash will slide left, but the balloon will move to the right.
Of course none of this is really any more strange than the fact that the balloon rises instead of falling when released. But I find it strange and
fascinating to actually see it do these things in an accelerating car.
The phenomenon obviously has a relation to the principle of equivalence, but in detail seeing how it works out takes some thought. The POE says that
an accelerated frame is equivalent to a gravitational field in its physical effects. This is fairly trivial when applied to Newtonian mechanics but
leads to nontrivial results when applied to electromagnetism, where it predicts that light is bent by a gravitational field. But the basic idea is,
if balloons rise in a gravitational field, and if an accelerated frame is equivalent to a gravitational field, then the ballon should move in the
direction opposite the effective gravitational force (eg, toward the center of curvature in circular motion, like when making the right turn in the
car).
In more detail, the fact that the balloon rises in an atmosphere of a heavier gas in a gravitational field can be regarded as due to the fact that the
air pressure is higher lower in the gravitational field than higher up; therefore, the force from the air pressure pushing up on the bottom of the
balloon is more than the force pushing down from the top. If this difference is greater than the weight of the balloon (assumed here) then the
balloon rises.
If the car were sealed so that it could not exchange air with the outside, then when you turn the corner the effective gravitational field causes a
gradient in the air pressure in the car, with higher pressure on the side opposite the center of curvature (in the case of the rotating frame, the
right turn of the car). And the pressure difference between the two sides of the balloon moves it.
Of course, the car is not sealed, so air is coming in or going out of the cracks, responding to pressure differences. This obviously complicates the
situation, but the fact that the balloon responds as it does means that the pressure gradient actually does exist inside the car.
I just thought this was so interesting and easy to see that I wanted to share it.
[Edited on 4-3-2015 by annaandherdad]
Any other SF Bay chemists?
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Zombie
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No way!
I just tried this on my hardtail, and the balloon is gone.
I'd have to see this in action.
If you were in a plane, and taking off... The balloon would go to the cockpit?
They tried to have me "put to sleep" so I came back to return the favor.
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careysub
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Quote: Originally posted by annaandherdad  | ...The phenomenon obviously has a relation to the principle of equivalence, but in detail seeing how it works out takes some thought. The POE says
that an accelerated frame is equivalent to a gravitational field in its physical effects....
[Edited on 4-3-2015 by annaandherdad] |
I have encountered similar statements in the non-technical literature about the Einstein Equivalence Principle (there is the weak form from Galileo),
but I think that the description may be incorrect.
The statement in Wikipedia under Einstein Equivalence Principle is I think correct:
"The outcome of any local non-gravitational experiment in a freely falling laboratory is independent of the velocity of the laboratory and its
location in spacetime."
It is really a statement about an unaccelerated frame of reference, sort of the inverse of the gravity and acceleration equivalence description.
A key reason that the "accelerating" (as opposed to free-falling) elevator description is inaccurate is that in all of the cases encountered in the
real Universe you can tell the difference between gravity and acceleration. An instrument that will do this is called a differential gravimeter or
gravity gradiometer.
In an accelerating elevator frame of reference the strength of acceleration is everywhere the same. In a real gravity fields the strength of the field
drops with distance from the center of mass, even taken at a mathematical point there exists a gradient in the gravity field that does not exist in
the accelerated FOR.
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Morgan
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A balloon demo tidbit.
A Baffling Balloon Behavior - Smarter Every Day 113
https://www.youtube.com/watch?v=y8mzDvpKzfY
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Zombie
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Thanks for posting that Morgan.
The simple explanation makes sense, ie: heavier air displacement.
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Oscilllator
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This effect can be demonstrated without use of a car using a spirit level of the kind that builders use. the bubble of the spirit level moves
backwards as you accelerate the level forwards.
I used one of these to demonstrate this effect to my physics teacher, as she (and the rest of the class) didn't believe me.
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Zombie
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It is a slick effect.
Reading the long explanations I was still shaking my head.
Put me in a car going to get candy??? I got it instantly
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Molecular Manipulations
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You're wrong. I just tried it on my motorcycle, the ballon just went up. Then in my convertible, same thing. Afterwards I did it in my pickup, it
worked just like you said. This is why I'm failing physics, it's just so damn inconsistent!
[Edited on 6-3-2015 by Molecular Manipulations]
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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Zombie
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It's the black hole in the center of our galaxy.
It sucks up all the balloons. The roof of the truck provides a gravitational shield.
Please don't ask for Citations. My dogs ate them.
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Molecular Manipulations
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That's a good explanation, helium balloons act like quantom particals, they don't follow Newtonian physics like the old air balloons do. That's why
I'm investing in helium, when the rest of the universe gets sucked into the singularity, my balloons will be orbiting the black hole just
inside of the event horizon.
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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Zombie
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Bring beer!
You will have the best vista in the galaxy! 
They tried to have me "put to sleep" so I came back to return the favor.
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Dr.Bob
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I had a college roommate that liked to explain to non-scientists that the reason light bulbs (the old incandescent kind that did burn out often)
stopped working is that they would fill with darkons, which are the particles of darkness that make it dark. Once the darkon collector (bulb) was
full, it could not collect darkness any further, and thus stopped working. He convinced a number of people (many were blond females...) of that,
many of whom likely baffled their physics teachers later in their studies. I always thought it was an amusing concept, and still mention that when a
bulb burns out, which is rarer now. Almost as good as the petitions to ban dihydrogen monoxide.
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Molecular Manipulations
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And the only vista in the non-existent universe! I guess the balloons can hold up a can or two, absolute EtOH would be more conservative.
Maybe I could float on the Blazar's relativistic (astrophysical) jet, that would be a sight...
-The manipulator
We are all here on earth to help others; what on earth the others are here for I don't know. -W. H. Auden
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Zombie
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Bert posted up a long explanation of how "Dark Suckers" work.
It was funnier that hell.
He convinced me it's true!
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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Praxichys
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Someone needs to add weights to a helium balloon until it is just barely floating, and then take it into an elevator.
Then watch as people are baffled when the balloon sinks when the elevator descends. Explain about the darkons, etc.
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Zombie
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| Quote: Originally posted by Praxichys | Someone needs to add weights to a helium balloon until it is just barely floating, and then take it into an elevator.
Then watch as people are baffled when the balloon sinks when the elevator descends. Explain about the darkons, etc. |
Now that you mention it, I have seen that happen in the Trade Center elevator.
My daughters B'day.
Funny the things that come back to you.
They tried to have me "put to sleep" so I came back to return the favor.
Zom.
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