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Balloon in a Car

 
 
 
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beero1000

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Physics question: You have a helium balloon in a car. The car begins accelerating, what happens to the balloon?


 
When my brother and I were kids our dad tried to demonstrate this effect for us. He accelerated the car forward and the helium balloon swung toward the rear of the car, just like the heavier-than-air pendulum in the video. Much analysis and experimentation ensued...

Left as an exercise for readers: what went wrong?
 
Bomb#20 said:
When my brother and I were kids our dad tried to demonstrate this effect for us. He accelerated the car forward and the helium balloon swung toward the rear of the car, just like the heavier-than-air pendulum in the video. Much analysis and experimentation ensued...

Left as an exercise for readers: what went wrong?
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This is really a demonstration of buoyancy. If your balloon went backwards, I would suspect rear vents in your car prevented an increase in air density, which is what pushes the balloon forward.
 
That is a pretty cute plug for an audio book and a means to raise funds for sending those cute kids to further education.

No flies on that young father. There may be hope for the future generation yet, lol... :D
 
seyorni said:
Window/s open?
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Bronzeage said:
This is really a demonstration of buoyancy. If your balloon went backwards, I would suspect rear vents in your car prevented an increase in air density, which is what pushes the balloon forward.
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Interesting proposal -- somebody should try it and see if opening windows changes the outcome. That wasn't the problem we had though -- we eventually got the demonstration to work without sealing/unsealing the car. No, what we were doing wrong was we didn't have the balloon anchored by a string from the floor like in the video. It was just resting against the ceiling of the car.

Left as an exercise for readers: why did that screw up the results?
 
Bomb#20 said:
Interesting proposal -- somebody should try it and see if opening windows changes the outcome. That wasn't the problem we had though -- we eventually got the demonstration to work without sealing/unsealing the car. No, what we were doing wrong was we didn't have the balloon anchored by a string from the floor like in the video. It was just resting against the ceiling of the car.

Left as an exercise for readers: why did that screw up the results?
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Static cling and/or resistance of the rubber against the roof of the car would be my guess.
 
Bomb#20 said:
Left as an exercise for readers: why did that screw up the results?
Click to expand...
How a fluid sloshes around within a vessel is affected by interior shape, barriers, openings in the vessel, &c. Not everyplace will be in the path of a current.
 
Well that explains why the &^%$%^ birthday balloons are always determined to float forward and block my driving view all the time. :p

Very kewl dad! Lucky kids. :)
 
The balloon keeps doing what it was always doing - moving away from the applied force.

When the van is stationary, the balloon is trying to move AWAY from the acceleration due to gravity - that's why it doesn't simply fall to the floor, right? - add a horizontal component of acceleration, and the balloon continues to move away from the acceleration - ie both up and forward. The pendulum moves towards the source of the acceleration - ie down (when stationary) and both down and backward when accelerating in the horizontal plane.

An untethered balloon is not part of the accelerating system at all; it just stays where it was, and the car moves forward without it; from the POV of the passengers, it moves backwards. (in fact, it moves forwards quite a bit, due to friction with the roof of the vehicle, and due to the forward motion of the air inside the vehicle; but the car accelerates forwards faster than the balloon).
 
bilby said:
The balloon keeps doing what it was always doing - moving away from the applied force.

When the van is stationary, the balloon is trying to move AWAY from the acceleration due to gravity - that's why it doesn't simply fall to the floor, right?
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Nah. The balloon floats on the air. Both the balloon and the air is pulled down by gravity. But the density of the balloon is less than of the air so the balloon is pressed "up" by the air that "wants" to get under it.
 
Juma said:
Nah. The balloon floats on the air. Both the balloon and the air is pulled down by gravity. But the density of the balloon is less than of the air so the balloon is pressed "up" by the air that "wants" to get under it.
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That is true, but changes nothing. The effect of the buoyancy is exactly what I wrote; you can ignore the fact that the air is the source of the forces, and just treat the balloon as 'wanting' to accelerate in the opposite direction to any applied force. It achieves the same result, but is a lot simpler.
 
bilby said:
That is true, but changes nothing. The effect of the buoyancy is exactly what I wrote; you can ignore the fact that the air is the source of the forces, and just treat the balloon as 'wanting' to accelerate in the opposite direction to any applied force. It achieves the same result, but is a lot simpler.
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Think of how your explanation would work if we remove the air (think of the moonlander).


Your answer doesnt explain anything. Its like this explanation of how a plane can fly that once was given in a swedish tv program for children: there is air under the wing and "there is air above the wing and that is why a plane flies".

The interesting thing is "what makes the balloon move and why?" And the answer is not that the ballon want to escape gravity.
 
Juma said:
Think of how your explanation would work if we remove the air (think of the moonlander).


Your answer doesnt explain anything. Its like this explanation of how a plane can fly that once was given in a swedish tv program for children: there is air under the wing and "there is air above the wing and that is why a plane flies".

The interesting thing is "what makes the balloon move and why?" And the answer is not that the ballon want to escape gravity.
Click to expand...

If you remove the air, the driver dies before he can make the car accelerate.

In the reference frame of the original question, my answer is simpler than yours, and still gets the right results; it is a more elegant solution, but less broadly applicable. Which of these is more important is determined by the future applications we may want to develop using this knowledge. For pure science, your solution is better, because science is not limited to the surface of the Earth; For engineering applications, which is best depends on the specific application; and for everyday human life, my explanation is better.

Given the casual nature of the observations, the results are insufficient for use in pure science as they stand; "Hey, the balloon moves forward" is cute, but more detail is needed to make it useful for any more practical issue than deciding how to prevent balloons in a car from distracting the driver.

My answer explains everything yours does, except that I omitted to mention the fact that less dense objects float in more dense fluids. A fact that frankly doesn't seem sufficiently counterintuitive as to be worthy of mention.
 
bilby said:
My answer explains everything yours does, except that I omitted to mention the fact that less dense objects float in more dense fluids. A fact that frankly doesn't seem sufficiently counterintuitive as to be worthy of mention.
Click to expand...
Sorry, didnt mean to insult your ego.
 
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