Helium Balloon: Newsletter Challenge (07/06/10)

The Helium filled balloon will remain where it was before the car accelerated, because the balloon is filled with a very light gas that has a strong tendency to go up vertically hence, as the question describes that the balloon is pressing against the ceiling.

I think your all missing the point, the balloon moves because it wants to!!!

I was considering the other day, if the results would be different if the balloon itself I (I mean the rubber material) were heavy enough. Well, I found out that the balloon will move forward, irrespective of the balloon weight:

First of all we know that the balloon is stuck on the roof, therefore, the upward buoyancy is greater than the weight, i.e. m_a.g > m_b.g + m_h.g , where m_a is the displaced air, m_b is the weight of the balloon and m_h the weight of the helium inside the balloon. Exactly the same equation holds in the case of the horizontal acceleration, by substituting g with a. In the end, all boil down to the following constraint to have buoyancy which is greater than inertial force:

m_b > m_a - m_h

so, if the balloon is stack on the roof in the first place, then it will move forward in a possible acceleration of the car, whatever the weight of the balloon might be.

Good answer tkot. I was wondering if a weak acceleration might be unable to overcome the static friction at the ceiling, which would prevent the balloon from moving forward. But on reflection I see that taking moments about the attachment point show that the buoyancy force through the balloon center would cause the balloon to roll forward despite the friction at the ceiling. It does not have to slide. That assumes a round balloon. One with an odd or flat shape against a rough ceiling might be unable to roll or slide under weak acceleration. So for a round balloon I think your conclusion is correct - if it floats to the ceiling it has to move forward under acceleration.

OK I made some stupid mistakes due to rush. Of course I meant that m means mass and not weight, and certainly it should hold: m_b < m_a - m_h in order for buoyancy to win. (I repeat that the index b stands for balloon, a for displaced air and h for helium.)

Anyhow, if we take into consideration the friction, as you say, then indeed a small acceleration might not be enough to move the balloon due to static friction, IF the rotation is not an option. Just for fun, and in order to remember our early school physics, I will try to device a better formula that additionally takes account of the friction:

Say that μ is the static friction coefficient between the balloon and the ceiling. The force with which the balloon presses against the ceiling is the difference of the vertical - this time - buoyancy and the weight, i.e. m_a.g - (m_b+m_h).g = (m_a-m_b-m_h).g

The horizontal force is the combination of the horizontal buoyancy force F_b=m_a.a , the inertial force F_i=-(m_h+m_b).a and the friction F_f=-μ.(m_a-m_b-m_h).g (we assume that the buoyancy wins so movement tends to be towards the front.)

The net force is therefore: F=m_a.a-(m_b+m_h).a-μ.(m_a-m_b-m_h).g and in order to have movement towards the front of the car the constraint for the acceleration a is ... guess what ... a>μ.g

Again, the balloon mass or volume or whatever is canceled out.

Maybe this is not a surprise, as the horizontal and vertical axes are symmetrical, i.e. Newton laws and buoyancy mechanisms are the same in both directions.

Nevertheless, in case rotation is allowed (as it is certainly true in the real world), then there is nothing to prevent the balloon from rotating even at an infinitecimal acceleration. There will always be a torque to cause this effect, unless the balloon is stringed somehow.

tkot - Isn't the sign of your last inequality wrong ? The balloon mass should be less than ma - mh for buoyancy.

Well, I found out that the balloon will move forward, irrespective of the balloon weight:

Perhaps it is only your use of English, but clearly, if the balloon material is heavy enough, the balloon will not float. 25 grams mass added to the sort of helium balloon that might be found in a car is enough to make the balloon non-buoyant in air. Then such a balloon will not move forward under car acceleration. (A basketball filled with helium would be an example of a non-buoyant helium balloon.)

so, if the balloon is stack on the roof in the first place, then it will move forward in a possible acceleration of the car, whatever the weight of the balloon might be.

Although your inequality seems incorrect, even without any math, it should be completely obviously to anyone that if a balloon is buoyant ("if the balloon is stack on the roof in the first place") then it will act buoyant ("it will move forward in a possibile acceleration of the car").

If there is a point in this post, I've missed it.

When the car accelerates the air into the car and balloon feels the effect of a force field in the opposite direction of the car acceleration. Since the helium into the ballon has a density smaller than the air, as the principle Archimedes says the balloon feels a force opposite to that of the force field caused by the car acceleration that is in the same direction of the car motion.

Because of the tear dropped shape of the balloon the point where contact with the roof of the car is at it's apex with the heaviest part of the balloon (the knot) in a plum alignment directly beneath it. Think of the basket below a hot air balloon. When that basket starts swinging in an arched fashion like a pendulum there is a point where the centrifugal force of the swinging mass (the basket) will pull the balloon downward. This is just before the point where the mass becomes weightless and starts it's return swing. At this point the mass hanging below the balloon will free fall straight down and the balloon will ascend very rapidly straight up until the mass is directly under the new center of gravity causing a whipping effect like a catapult sort of. Now if the balloon is up against the roof the pivot point is where it contacts the roof and is the center of gravity. When the car accelerates it shifts the center of gravity and the heavier mass hanging beneath that center of gravity starts heading for the rear but the balloon restrains it's motion to an arch. This creates a centrifugal force that will break the contact with the roof as it pulls it downward. This in combination with the point of weightlessness that is experienced at the top of the arching motion the balloon suddenly shifts the center of gravity and it whips the mass forward with inertia greater then what it would have generated otherwise. This forward momentum causes the balloon to travel to the front of the car. This shifting of gravitational centers doesn't have to be some major shift as it may hardly even be visible.This would be the case especially if the balloon has a string tied to it. A sudden violent acceleration could send the balloon tumbling though. I hope this explains things clearer then my earlier postings.

Not enough info, define "pressed". "Pressed" at what force? If the "pressed" force is great enough to overcome the pressure gradient caused by acceleration the ballon is part of the ceiling and does nothing. You have to make assumptions of data not in evidence.

I was able to read about 114 posts through, so I am not sure if anyone has agreed on what will happen. One poster said it will depend on many factors. One factor I think we must consider, is the the diameter and shape of said balloon. Is it more, or less, than half the height of the interior height of the car. Is it one of those balloons that a clown has made at a child's party? (the ones that look like animals?) To my knowledge, even a round balloon is not perfectly round. What is the volume of the balloon versus the volume of the interior space of the car and how are the seats arranged in this car - just one driver's seat or no seats at all. Another factor, would be the buoyancy of the system called a balloon (material plus the weight of its current internal volume). Ironwood sinks you know. A balloon that is not filled with enough helium stays on the ground. What is this balloon's initial "buoyancy"? Does it just barely float? The word "pressed" in the given, would seem to allude that this buoyancy is significant. But, how "hard" is it "pressed"? Was taped used to "press" it to the ceiling? Or, did the balloon "roll" around on the ceiling before you stopped for the light. Static force ... anyone? How humid is this air? Is it dry & cold, hot & humid? Consider a closed bottle filled with foam (water and dish detergent) if you are not sure why this matters. Raise your hand if a light just went off in your sensory unit and have heard of cohesion. Which force(s) did I leave out? Tell me this is not uhmmm "normal". Do you accelerate up a hill or down a hill? And are we at sea-level in this car with a very large interior or is this a subcompact? What's the relative volume of the balloon in regards to the car's volume. Is anyone studying inertia "at the moment?" If not, then ask Mister Fantastic. Shockingly, he has dealt with this; and, his time may be flexible enough to torque up an example. How's that sound. Hope I'm not rubbing you the wrong way and you are getting all hot and bothered about it. I might be able to tell you how to cool down so you can transfer that heat and not get all inflated on me. I probably left some out. That sucks more than an airfoil. -------------------------------- How many forces can you assume negligible? I'd guess one or two for every assumption you make of these 2 systems. The helium container and the air container that contains a helium container. The first assumption I would make, is the air container is much more rigid than the helium container - the windows are closed and it is not a convertible with the top up. -------------------------------- Define a compressible system's density, as proportional to the SUM of the forces imparted to it with respect to a change in volume. Define Pressure as a force of mass with respect to it's change in velocity. Now: Consider a compressible system (other than a black hole) held within a boundary - be it due to cohesion, weak force, or "whatever" FORCE & perhaps, the normal force of a container (air in the confines of a car). Consider another system (balloon) displaces its weight in this denser compressible system (or even a non-solid, non-compressible system) until it displaces enough weight and will move(float) opposite the SUM of the vector forces applied to it until it meets another system (or part of a system) that it cannot displace. (Did I let the cat out the bag?) Can you tell me why a nearly round helium-filled balloon, pressed up against the center of the ceiling, in the middle of an accelerating car, on a level surface, who's orientation was horizontal, with respect to the horizontal ground, throughout the period of acceleration, would not initially vector down, in the direction opposite the change in speed, in a clockwise motion, when viewed from the passenger's side of a car, who's steering-wheel is on the left? Only if you answered ... because, it's normal, buoyant force was great enough to make all the other forces negligible - after all helium has negative apparent weight in air - would I believe you. In which case, it would roll on the ceiling in the direction of acceleration, rotating clockwise as viewed from the drivers side, until it reached the opposite vector of the combined forces of gravity and the car's acceleration, if the force of air resistance was not significantly greater than the force of friction of the ceiling. If the frictional resistance of the ceiling was nearly negligible with respect to air resistance, it might roll counterclockwise in this instance.

I thought of an interesting variation. Suppose you tie a string to the balloon and hang a carefully selected weight on it so that the ballon+string+weight has neutral buoyancy. You leave this floating somewhere in the middle of the car (ok, you might need a van if it's a long string). Now what happens when you accelerate? My prediction is that the balloon+string+weight will not move bodily in any direction but will reorient itself with the balloon forward and the weight backward.

After reading partway down the list of replies, hasn't anyone actually witnessed this? I have. Our daughters are grown now, but we've transported helium balloons a number of times.

The balloon floats forward during acceleration, rearward during deceleration, and towards the inside of a curve, because it is floating in the heavier cabin air which is affected to a greater extent by the accelerations and gravity. The balloon therefore moves opposite the G forces.

Easier than that ... get any bottle with a bubble in it and push it along it's length and watch the bubble move inside the bottle in the direction you push it. :o)

I think you will enjoy this explanation ... I can make it interestingly complicated and intuitive at the same time! ... If you consider this ... Since Helium has a negative apparent weight with regards to air (air sinks in helium) ... place air in a balloon whose material is lighter than helium ... Place this air filled balloon on the floor of your car which is filled with helium. Accelerate said car ... Which way will the balloon go? Now rotate you answer 180 degrees ... zonks!

The balloon should move forward, as the horizontal acceleration makes it appear that a perpendicular gravity is facing backwards, and the balloon will move in the opposite direction (forward). Another way of saying this that the combined effective acceleration fields (gravity + horizontal accel) "tilts" the gravity vector backwards, setting up the aerostatic pressure gradient to have a horizontal component. The helium is buoyant within an aerostatic gradient, and moves accordingly.

About the same thing as when you accelerate a spirit level - the bubble is displaced forward.

The big laugh - when that came up months ago - was Bob had lost his bubble.

There are 2 separate and new forces exerted on the balloon when the car moves off.

The first is friction from the roof of the car, and the opposing force pushes backwards, the second is inertia, and the reaction also pushes backwards. In the case of the balloon at rest, the center of gravity is lower than, and amounts to less than the center of lift. As soon as a rearwards reaction acts on the difference between these two forces, the greater force acts on the unit.

This means the lifting force (greater than the gravity force) has control, and forces the balloon forward if it can overcome the friction force, otherwise it will just roll forward until the gravity cancels the lift.

the ballon moves forward relative to the center of the ceiling, because it is floating implies that it has less mass per area than the air around it. This means that the air in the car has a greater resistance to motion. When the car moves forward the air being a compressable fluid will bunch up in the rear of the car. This bunching up will make less room for the lighter than air ballon thus pushing it towards the front of the car.

Assuming the windows and doors are closed, AC off, and still air, the balloon will move forward, because the cars acceleration will compress the air at the rear of the car pushing the balloon forward.

A helium balloon behaves like an object with negative mass and it does just the opposite of objects with positive mass, so when the car speeds up the balloon moves forward and when the car slows down the bolloon moves backward.

We can ignore the effects of gravity because the ceiling of the car constrains the balloon to move perpendicular to the direction of gravity. That leaves the inertial force due to the car's acceleration, which points, by definition, in the opposite direction of the car's acceleration: toward the back of the car. Because the balloon is less dense than the air, it will experience a buoyant force in the opposite direction of the inertial forces acting on the balloon-air system. The buoyant force will thus act to move the balloon toward the front of the car.

The balloon should bump across the inside roof of the car forward towards the windshield.

Taking the positive x axis as the direction the car is facing, all fluid (air) in the cab of the car will exhibit the same negative acceleration. The helium filled balloon, being less dense than this air, will float in the positive x direction towards the front.

The acceleration of Earth's gravity will continue to cause the balloon to remain constrained against the roof of the car.

well I think its a two part answer. 1 the density of the two gases, 2 the simulated gravity created by the forward acceleration.
How: The forward motion causes the weight of the molecules to sightly compress to the rear (Atmospheric pressure due to gravity) the Helium being lighter has less pull from the acceleration or gravity, and is displaced by the heavier gas moving towards the back of the vehicle forcing the balloon forward.

It moves forward. It's like a less dense object in a centrifuge.

Hi Guest (comment 212)

"It moves forward. It's like a less dense object in a centrifuge."

Exactly;

You seem to have a clear grasp of physics and efficient, pertinent and illustrative expression skills.

There are quite a few useful resources members enjoy and guests only see the tip of the knowledge, information and communication iceberg.

Please consider registering.

34.5

I'm guessing the balloon will actually drop down slightly and bounce slightly towards the back of the car. As the car moves forward, the balloon will tend to remain stationary but will be lightly touching the roof. This should result in the balloon taking on some rotational momentum but as the balloon shape will resist change (there should be a slight flat spot where it touches the roof) it should I think push itself off the roof and bounce away towards the back of the car.

Tom.

baloon moves to the back of the car, according to Newton law

This would be an excellent time to get a balloon and test your theory

It all comes down to mass and friction. A balloon filled with helium would have a mass less than the air it is floating in and there would be enough friction between the ceiling and the balloon that it would be stuck to the ceiling just as a weight heavier than air would be stuck to the floor. Since the air is moving forward with the car, the "compression" of the air from acceleration would be insignificant. If is no significant pressure pushing the balloon back due to its relative mass, the friction with the ceiling would be enough to overcome any tendency to be pushed back with acceleration and it would not move relative to the car.

Air motion as this relates to air (gas) density will cause the helium vessel (balloon) to travel to the lower pressure location. Therefore as you accelerate the surrounding vehicle air will compress in the back of the cabin causing the helium to 'leap' forward. In deceleration the opposite will occur. Lateral movement of the vehicle will cause movement of the helium to also be in the opposing direction to the direction of acceleration (until equilibrium of pressure is achieved allowing the balloon to centralize again).

How big is this balloon? What is the gas pressure inside the balloon? What altitude is the experiment conducted at? Is it a woman driver? Not enough information to form a valid answer.

what?

As long as the car windows are closed to prevent wind turbulence, the ballon will go forward. As helium is lighter than air the density of air at the back of the car will increase when the car moves forward as the air wants to remain where it was, providing a density gradient where the lighter than air balloon will move to the lower density at the front of the car. The same would be true if stopping, the balloon will go to the back of the car as the density increases at the front.

PT London, Ontario.

I'm an entry-level engineer and a very visual thinker. Here is my answer to the question, though I have never actually observed this.

At the moment of initial acceleration and as long as the rate of acceleration is increasing, because of inertia and the fluid-like motion of air moving to put more pressure at the rear of the vehicle, the balloon will move toward the rear of the car. The amount it will move will depend on the length of the inside compartment of the vehicle, if the windows are open and where, how full the balloon is, the material the balloon is made of, and many more factors.

When the rate of acceleration stops increasing, the air pressure is greatest at the rear of the car as compared to the front, which will cause the balloon to want to move toward the front of the car. If the ceiling of the car had a low enough drag, the balloon would start sliding toward the front with the bottom of it still facing toward the rear of the vehicle. More than likely, in a typical car, the balloon would just rock or "jump" a little forward. If this were a very powerful car, the pressure difference might be great enough to cause it to overcome the friction and move to the front of the car. Of course, if the car stops accelerating, the balloon will return to a resting position at wherever it has stopped on the ceiling.

Bradley (The original BLiP)

P.S. This is fun!

If you had a Mustang like mine, you could accelerate at 9.8 m/s^2, and the balloon would start acclerating at a 45 degree angle toward the front. Since the ceiling would prevent the vertical component of the 45 degree angle acceleration, the rate of forward acceleration would be determined by the roughness of the ceiling.

I still maintain that the instantaneous movement of the balloon would be to stay at rest with respect to the earth.

Working on the whole density and inertia ideas then the air near the floor of the vehicle is cooler, denser and therefore has greater inertia than the warmer air near the roof. As you accelerate this cooler air will move to the rear of the vehicle thus causing the warmer air near the roof to flow foward. The inertia of the balloon compared to the air resistance is small and the balloon will flow forward with the warmer air.

I agree with you on this one. Many times when I drive, particularly in summer, I notice the air flow as I accelerate, brake, or corner (see my initial comment). The turbulence is somewhat disturbed by the chairs and other objects in the cabin, but is very noticeable. Though there has been a lot of impressive maths thrown around, much of which is useful and necessary for determining the multitude of forces acting on the said balloon, there has been none that I can see that has calculated the flow of air inside the cabin of the vehicle due to the denser cold air on the floor of the car rushing backward in the cab as acceleration takes place, which effectively pushes the warm air at the roof of the cab forward, thus pushing the balloon along with it in its flow. It would be almost impossible to work out the forces mathmatically, as there are so many variables. But flow rate is easy to see if you did a little experimenting.

Imagine attaching small light paper streamers (lets say 100mm long), to the roof of the car. They are heavier than air and hang down. Thefore, the theorists who are offering the wounderful maths would theorise that they would angle toward the back of the car as acceleration takes place. This would be true if all the air in the cab was at the same temperature and there were no obstcles in the cab to disturb the air as it all "compresses against the rear windscreen". But as this is never the case, the warm air which is pushed forward by the backward flow of cold air on the floor, would also push these 'heavier-than-air' flags forward.

If these small flags were set in an evenly spaced pattern over the roof, windows and walls of the cab (and for laughs include the seats, driver and passengers in the vehicle) then they would each demonstrate that airflow inside the cab is totally dependant on the aerodynamics of each of the objects and the interplay of the eddies and flows that are created by the movement of air over, around and past these disruptions to the smooth flow of the air as it is acted on by the forces of gravity and acceleration.

If the cab was perfectly spherical, with no internal objects to disrupt the airflow, then if the horozontal acceleration was equal to gravity, the coldest air (sitting at the floor) would flow backward in the ball (like a pendulum) pushing the warmer air forward at the same rate, until it rested in equalibrium at 45deg to the horozontal direction of acceleration (ie half way between the force of gravity and the force of acceleration). If a helium balloon was placed inside the ball of air, it too would flow forward with the warm air. But as soon as other objects are introduced into the ball (cabin) the flow is disturbed and will therefore become more difficult to predict.

Moves forward, toward you, because the mass of the air as you accelerate attempts to move to the back of the vehicle, due to inertia. The result is that the balloon, being lighter than the mass of air, moves forward, as it is displaced by the mass of the air moving back.

The balloon moves forward due to the density of the air that moves back due to the acceleration of the vehicle. The balloon is displaced by the volume of air transfered to the rear.

The balloon moves forward to a point directly in front of your face, effectively blocking your view of the road.

The reasons this translucent floating party favor tries to kill you can be explained without delving into the the obvious demonic qualities of helium-filled balloons (we've all heard someone speak after inhaling one).

So I propose that it primarily due to airflow. Airflow created when the cooler/denser air near the floor of the vehicles cabin is force to the rear upon acceleration, displacing the warmer/lighter air at the roof and moving it forward. The air moving forward around your head creates eddies that the balloon finds particularly inviting and unless you have a pin handy or lightning fast fingers on ergonomically designed a/c blower controls, you will have to battle them with your bare hands to regain a clear view of the road. (Extremely dangerous when multiple balloons are involved!)

The balloon will move to the front of the car, i.e. the front window.

The density of the air in the car is higher then the density of the helium, the air in the car will therefore force the balloon to the front of the car.

Regards,

Martien Versleijen

Your car, the air in it and the helium filled balloon are all at rest. Stepping on the accelerator begins to move your car forward. Due to inertia, the mass of air in the car initailly wants to stay where it is in relation to the ground so the car moves forward somewhat independantly of the contained air mass thereby increasing the air pressure and density in the rear of the passenger compartment and lowering the pressure in the front. The balloon containing low density helium is forced forward into the lowest density air at the front of the passenger compartment in order to best maintain pressure equilibrium.

I think the answer depends on two things:

1) Does the balloon have the typical "mouthpiece" and knot, causing one side of the balloon to weigh more, or a non-spherical shape (despite the total weight being less than air)?

2) The coefficient of friction between the balloon and the roof.

If the balloon we perfectly symmetrical and there was a coefficient of friction between the balloon and the roof, I would expect the balloon to initially start to roll forward as the stationary air in the car got compressed against the rear window as the car accelerates and the less dense balloon then moves forward.

If the balloon is not symmetrical, the result might differ. In the case where 100% of the mass of the balloon was at the knot, then on accelerating, there would be a net torque (air pressure acting on the center of pressure, acceleration acting on the knot, which might result in the balloon not moving or even the bottom rotating to the back. If there were no friction, then balloon would still move forward, but would rotate so the knot were to the rear.

There are two things going on here. the first is Newtonian force, the second is the molecular-level interaction between the balloon and the air surrounding it. To illustrate the first, let us replace the air in the car with a vacuum (naturally giving the driver a NASA space-suit). The balloon will now be lying on the floor (the seats have been removed), and as the car accelerates the balloon will experience a tangential force due to friction between its skin and the floor. This is the ONLY change produced by the acceleration. I hope no-one will dispute that the balloon will roll towards the rear of the car. Re-introducing the air, and moving the balloon to the ceiling does not change this - the balloon will still tend to roll across the ceiling towards the rear, with exactly the same (tangential) force acting upon it at the point of contact with the ceiling.

However, now the air in the car comes into play. Remember that air is really composed of relatively few molecules and 99.99??? % empty space, with the molecules flying around at incredibly high velocities, bouncing off each other and the walls of the container. As the car accelerates forward, the rear "wall" of the car accelerates the molecules bouncing off it, and the front similarly decelerates the molecules bouncing off it. Because the velocity of the molecules is so many orders of magnitude greater than the velocity of the car this effect is felt instantaneously. So the molecules moving forward in the car are moving faster, and hence with more kinetic energy than the molecules moving backward. This will produse a net force on the balloon in the forward direction as they bounce off its skin.

Which force is greater? Because this is a trick question, it must be the molecular-level one, since that is almost counter-intuitive. Quad erat demonstrandum!

Don't forget friction & inertia

The balloon remains a balloon.

Because that what balloons do best.

Next...........question.

I believe that the balloon filled with helium would in fact move forward in the cab due to height change and weigt transfer the car would normally experience in acelleration. Thus, the balloon would make its way up, and since the car drops at the rear in sudden motion and lifts in the front this expains the balloon moving forward. If you were to be at a stoplight and suddenly back-up the car in reverse the balloon filled with helium would move to the back of the car. Good question!

The balloon remains a balloon.

Because that what balloons do best.

Thank you CI - you have restored my faith - welcome to GR4 you understander of physics you. Please accept bonus GA

If we walk through the free-body diagram and sum the forces in the x (horizontal) direction we find the ONLY force on the balloon in this direction to be...

Fx= density of air x balloon volume x acceleration of the car

Dividing by the mass of the balloon we get...

acceleration balloon = (density of air / density of balloon) x acceleration of the car

So we can see that there is no maximum acceleration per se, it is only proportional to the ratio of the densities and the car's acceleration. If we look at the equation it makes sense according to our experiences... that is, vary the density of the balloon. If we let it get to a neutral buoyancy it accelerates at exactly the same rate as the car. And if we let the density get larger than that of air it will move with less acceleration than the car (such as a ball rolling backwards on the floor!)

As a side note, I did make an assumption. The max acceleration occurs when the balloon is at rest (when there is no air resistance). Don't forget in the real world there will be air friction, friction from the normal force on the roof, and possible electrostatic force.

so bigger the volume and so less the mass of the balloon and included helium and so denser the environmental air so higher is the acceleration of the balloon

That's precisely it. Just as previous posts have pointed out, by doing the spirit level experiment a more pronounced effect can be seen because of a larger ratio of the densities.

the highest acceleration is if the balloon has an infinite volume and a mass of zero

good one!

like interplanetary/interstellar space

As this is the first question that I have responded to, I am unsure of the protocol for replying to the answer given by the questions supplier. I trust I am not speaking out of turn here in responding in this way.

I believe that a few issues have been ignored in the short answer provided by the questions supplier.

If the internals of the car cab were a smooth sphere with no objects that would interupt the smooth flow of air in the cab and if all the air in the cab was at the same temperature throughout the cab, then the outcome would be priety much as per the provided answer.

However, the veriables that exist in a 'real life' car cabin would bring about a similar result, but brought about by very different causes. I am not convinced that the provider did any experimentation to verify the accuracy of their answer. it seems they relied on physics taught in university, which would give us an answer identical to the provided answer.

If we were to introduce varying air temperatures in the air, which would happen in real life situations, then the hot air would rest with the helium balloon against the roof of the car while the cold air would be at the floor. As soon as we apply acceleration to the cab, both the hot and cold air have a tendency to compress against the back of the cab. The cooler air, being more dense than the warmer air, wins out and forces the warm air forward. If the cab was perfectly spherical, with no internal objects to disrupt the airflow, then if the horozontal acceleration was equal to gravity (9.8m/s^), the coldest air (sitting at the floor) would flow backward in the ball (like a pendulum) pushing the warmer air forward at the same rate, until it rested in equalibrium at 45deg to the horozontal direction of acceleration (ie half way between the force of gravity and the force of acceleration). If a helium balloon was placed inside the ball of air, it too would flow forward with the warm air, not beause the air is all compressing against the rear of the ball, but because the flow of the warm air is forward in the ball and drags the balloon along with it. But as soon as other objects are introduced into the ball (cabin) the flow is disturbed and will therefore become more difficult to predict. The shape of a real cab and internal objects in the cab mean that the airflow is considerably turbulant.

I suggested a little experiment in my second blog entry that would show that the airflow in the cab is not a simple matter of university physics. Perhaps it would be worthwhile if the questions supplier were to apply the experiment to see for themselves that their answer is rather simplistic, and is also misleading. The exact direction and speed of the balloon would vary considerably depending on the size and shape of the cab and what objects were in the cab to disrupt the steady airflow and create turbulance and eddies inside the cab.

I would appreciate having a reply from the questions supplier.

if you turn on the cars ventiulation, all these things can be forgotten

Great flying spaghetti monster, exasperation is reaching it's limit, I am ready to explode. This problem, as are all of these challenges, is idealized. Any "drivers" not mentioned in the question are not to be considered. Even a little child will tell you that a balloon that is at rest on the ceiling will move if you start a fan blowing air; that is, it will behave differently because you have changed the circumstances. If the questioner wanted the ventilation considered he/she would have said so in the question.

Well no actually - it complicates due to the relatively fine margins of density of a container of helium 'floating' in air - but "floating" means the base of buoyancy still operates - as it does for a bubble in a spirit level - or for a less dense particle in a centrifuge.

But what if; a stampede of pink elephants crushes the car?

Which way does the balloon go then - ay?

p.s. 'ventilation'

Perhaps the OP did not try the experiment, but thousands of observant fathers and mothers certainly have. When you accelerate, the balloon moves forward, relative to the car. When you corner, the balloon moves to the inside of the turn. This holds in normal everyday driving, with the ventilation turned on (but, obviously, less reliably with the windows open.)

Perhaps it would be worthwhile if the questions supplier were to apply the experiment to see for themselves that their answer is rather simplistic, and is also misleading.

The OP's supplied answer is anything but misleading, in my view: things work this way in the real, ordinary world. And even if that were not the case, one can make the sorts of ordinary assumptions that are made to answer these questions (as well as those in physics texts, which often leave out clear specifications for what is to be assumed).

It is a simple question with a simple, observable answer. For readers with some ability to intuit, it is the same question as "In what direction will the bubble move in a spirit level when you accelerate the level along a flat surface." In answering that question, it is not necessary to list all possible confounding factors, because just as in the balloon case, the reason the question is asked is not to engineer a system that will behave flawlessly in all conditions... it is simply to point out a phenomenon that, to some, seems counter-intuitive.

There are many times when over-analysis is counter-productive, making the "analyzer" appear both pedantic and far too literal-minded.

Fancy that - I didn't realize we have an "official answer".

_{(i must pay better attention, i must)}

Not misleading? Great fobs of folly. I wonder what the force is for the " it (air) rushes to the back of the car and forces the balloon forward." I would think that the back window would move forward, compressing the air in the back, while the front windshield would move forward, leaving a slight vacuum in the front. It is totally misleading to state that the air rushes to the back.

You have to view the answer in the context of all the other official answers for challenge questions. It is extremely rare that one is clearly and concisely phrased, technically accurate and technically complete.

In this case, the supplier of the official answer is viewing the "air" as an observer in the car, not as an observer outside the car. The gases filling the car's interior are air and helium. As the car accelerates, the air moves rearward, and the helium moves forward, relative to the car's interior.

There is no relative motion of the back window or front window with respect to the interior of the car. Whether the the air "rushes" (with respect to the car's interior) is debatable; perhaps "sloshes" would be better. But in any case, this is a simple and easily observable phenomena, analogous to the equally simple and easily observable phenomena of the bubble in a spirit level.

Acceleration simply changes the vector direction of "gravity", and the balloon floats accordingly, in keeping with Archimedes principals.

If the air were replaced by a bowling ball, many (most, I believe) people would say that the bowling ball rushes to the rear of the car when the car accelerates, rather than saying that the car moves forward relative to the bowling ball.

"You have to view the answer in the context of all the other official answers for challenge questions. It is extremely rare that one is clearly and concisely phrased, technically accurate and technically complete." . . .

. . .

"Acceleration simply changes the vector direction of "gravity", and the balloon floats accordingly, in keeping with Archimedes principals." . . .

MB -- Hopefully, most will realize your succinct and complete summary says all that needs to be said about the original problem and its solution. Any further "analysis" is "over the top" at this point.

Larry Fulwider

...so my head does get bigger when I press on the gas?

because the air pressure gets lower?