Balloon in a Car

[Bomb#20]

Left as an exercise for readers: why did that screw up the results?

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.

Makes sense; but that's not what was happening when we did it. For us the issue turned out to be that it isn't safe to "assume a spherical cow". Some parts of a balloon are buoyant and some parts are heavier than air. Naturally the most buoyant part rotates to the top and the heaviest part rotates to the bottom. When the balloon is resting against the ceiling of the car the contact point becomes a fulcrum and the entire balloon becomes a lever. The total buoyant force is of course greater than the total weight; that's why the balloon floats; but the heavy part is farthest from the fulcrum so it has a mechanical advantage. The heavier-than-air part of the balloon contributes the lion's share of the torque, so accelerating the car makes the balloon swing backward. We untied the string and cut off the excess rubber below the knot; then the balloon swung forward the way it's supposed to.

 

[ryan]

The balloon keeps doing what it was always doing - moving away from the applied force.

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).

This is just flat out wrong bilby. I don't think that you understood the explanation of the experiment. The air compresses in the back of the vehicle and pushes the balloon forward tethered or not. The tether's horizontal force just stops the balloon from getting to the very front of the car.

I think that you understand it now since you mentioned the buoyancy in a post after this, so why can't you just admit that you were wrong? This kind of behavior is so frustrating that I don't even like being on these forums anymore.

 

[bilby]

The balloon keeps doing what it was always doing - moving away from the applied force.

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).

This is just flat out wrong bilby. I don't think that you understood the explanation of the experiment. The air compresses in the back of the vehicle and pushes the balloon forward tethered or not. The tether's horizontal force just stops the balloon from getting to the very front of the car.

I think that you understand it now since you mentioned the buoyancy in a post after this, so why can't you just admit that you were wrong? This kind of behavior is so frustrating that I don't even like being on these forums anymore.

You are right; my comments about the untethered balloon were indeed wrong, for the reasons you point out.

My comments about the tethered balloon are still correct, however; in the stationary vehicle, the air compresses towards the bottom of the vehicle, due to gravity, and adding a horizontal component to the existing acceleration leads to the air compressing towards the rear of the vehicle as well; as a result, the tethered balloon always moves away from the direction of acceleration of the complete system. Juma's objection to my comments was that they were incomplete or misleading, rather than factually wrong, and my disagreement with him on that subject is based in our subjective assessments of how much can be taken for granted, and what elements of the problem deserve our focus, not on the facts of the situation. The direction of the force acting on the tethered balloon is always opposite to the force acting on the system. The behaviour of the balloon in this regard does not 'change' when the vehicle accelerates.

I have no problem admitting when I am wrong; but I do object to being browbeaten into admitting that I was wrong when my facts are right, but my choice of perspective fails to conform with the choices made by others.

 

[steve_bnk]

Start with an idealized first orderthought experiment.

1. A rectangular box the size of a typical family van.
   
2. Sealed and filled with air.
   
3. A light rigid ball filled with helium adjusted for neutral buoyancy floating at a point ¼ back from the front of the box on the mid point center line.
   
4. The box accelerates forward.
   

What does the balloon do?


First thoughts.

1. The air is compressed by the box rear as box moves forward.
   
2. A pressure gradient develops across the box front to back.
   
3. Ball moves forward.
   
4. If the box stops accelerating and stays at a constant velocity air returns to equilibrium, ball returns to initial position in the box.

Possible second order effects.

1. As air in back gets more dense it sinks creating a convection current at top and bottom of the box from lighter less dense air in front. If acceleration is maintained the ball may follow air currents.
   
2. Due to air friction the ball may not return to it its exact starting point in the idealized experiment.

 

[ryan]

The balloon keeps doing what it was always doing - moving away from the applied force.

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).

This is just flat out wrong bilby. I don't think that you understood the explanation of the experiment. The air compresses in the back of the vehicle and pushes the balloon forward tethered or not. The tether's horizontal force just stops the balloon from getting to the very front of the car.

I think that you understand it now since you mentioned the buoyancy in a post after this, so why can't you just admit that you were wrong? This kind of behavior is so frustrating that I don't even like being on these forums anymore.

You are right; my comments about the untethered balloon were indeed wrong, for the reasons you point out.

For what it's worth, I think this kind of honesty is inspiring, refreshing and noble.