Interesting Physics Question

[Atlantic]

there is an air-sealed elevator with a helium baloon inside held in the middle of the elevator.

The elevator wire is cut, and the elevator stats to free-fall down. Once he wire is cut and the elevator stated to fall, the helium baloon is released.

 

While the Elevator is falling the baloon will:

a.) Float to the top of the elevator

b.) Stay in the middle where it was released.

c.) Float to the bottom of the elevator

 

Factors: The elevator is filled with air, of which the pressure is equal to the atmospheric pressure outside (regular). Baloon is a regular rubber baloon filled with helium (like at birthday parties).

 

I don't know the answer myself, i was wondering what your views are. I took into account the motion of air particles in the elevator.

 

Looking forward to you replies.

[Atlantic]

I think the ballon will go to the bottom because when the elevator accelerates down, all the air is being forced up. According to the buoyancy it will go to a less dense medium, which is the low air pressure at the bottom.

 

Any of you braniacs have other opinions?

[fafalone]

It will float to the top. Very quickly. The pressure inside the elevator does not change in freefall, and its natural tendency is to move up. The only way it could go to the bottom was if the elevator was accelerating *very* quickly upwards.

[YT2095]

A) obviously

 

and it`s not gravity dependant either.

if the elevator was going UP the ballon would still rise, no matter how fast it accelerated, as the AIR being denser would also be forced to the floor, and the hellium being lighter would rise always above it on the air cushion. even if it were only nanometers away from the floor 

[Dudde]

dang YT

 

he beat me to it

 

but he's right

doesn't this go in the "phsyics" forum?

[YT2095]

I`de have thought so yeah, and Indeed I`de move it, but!.... oh hell here goes

 

 

 

---- time passes, many keys were pressed, the mouse was on fire, sweat was pouring, beer was consummed, shakes eventualy subsided, a reality pill wass consummed along with more beer, BIG GULP with fingers crossed ----

 

 

 

ok, consider it shifted

[VendingMenace]

wow, atlantic, what a fantastic question! Seriously, that is an excellent question.

 

Atlantic said:

I think the ballon will go to the bottom because when the elevator accelerates down, all the air is being forced up. According to the buoyancy it will go to a less dense medium, which is the low air pressure at the bottom.

 

excellent reasoning, but you forget that the balloon is also a free object, just like the air, and so the elevator would be continually "running into" it, in effect. So the ballon would be found more at the top. At least in the beggining, the ballon would appear to "float up". In the end, it would look just like everything else in free fall. Just my opinion.

 

The pressure inside the elevator does not change in freefall, and its natural tendency is to move up.

 

This is not good reasoning. Things tend to float up due to a density gradient formed by the gravitational field they are in. IN free fall, the air does not really expreience this, so to speak, and as such there is no density gradeint, and the balloon would not float up just becuase it "its natural tendency is to move up". Besides, in free fall in a windowless room, how would you know what was "up"?

 

A similare question would be, if you brought a helium balloon into an orbiting spaceship and let it go, where would the balloon end up or down or what? and why?

[YT2095]

Vending, the only reason the balloon would go "down" or downwards initialy, would simply be a function of Inertial momentum. allowed to stabilise you`de find that it would indeed float upon the air

[swansont]

There are two forces on the balloon - gravity and the bouyant force. Generally the bouyant force is larger and the balloon rises.

 

Accelerate the elevator, though, and you get a pressure differential which affects the bouyant force.

 

Do this in car, so that the horizontal bouyant force isn't fighting the vertical one, and the balloon moves forward when you accelerate the car forward, because of the air pressure buildup in the back of the car.

 

So for the elevator it will depend on what the initial bouyant force is and how that compares to mg. Do it with a neutrally or marginally bouyant balloon, and it'll move down.

[Kedas]

In free-fall you have the same effect as if there was no gravity.

 

The only reason the balloon goes up or down is due to the different weight of the gasses and it's own weight but if there is no gravity effect then there is no different weight for the baloon to move. So it will remain it's position.

 

(about the same like swansont said)

[YT2095]

with neutral bouancy in air, it would merely replicate it`s movements, but be a more "visual" representaion of it.

[fafalone]

If the magntitude of the acceleration was greater than the magnitude of the buoyancy force, then the balloon would hit the bottom of the elevator... it's not a massless object.

 

If you stand on a scale in a elevator and it says 1000N, and the elevator accelerates upwards, that reading will increase as you're moved down to the floor. The same would happen to the balloon, only it would take alot of upward acceleration (not velocity).

[YT2095]

yup, what he said

at a constant velocity all would stabilise as if motionless.

at a constant acceletation, pressure would be exerted upon the material within the lift, the pressure gradient being equivalent to the acceleration gradient.

[Kedas]

fafalone said in post #12 :

If the magntitude of the acceleration was greater than the magnitude of the buoyancy force, then the balloon would hit the bottom of the elevator... it's not a massless object.

 

If you stand on a scale in a elevator and it says 1000N, and the elevator accelerates upwards, that reading will increase as you're moved down to the floor. The same would happen to the balloon, only it would take alot of upward acceleration (not velocity).

 

The question says it's free-fall.

(your weight will be zero on your elevator scale.)

There is no buoyant force.

 

[VendingMenace]

lol at my original post, i had to run real fast to get to my class, and i didnt think through the original post well enough. That being said, here[ is what would happen...(i think)

 

In the begginning, we have a box filled with air with a balloon filled with helium in it. The balloon is held in place. Now the box is released into free fall and at the same instant the balloon is released as well.

 

We all know (thanks to gallileo) that all things fall at the same rate. Thus, the elevator, the air in the elevator and the balloon would all start to fall at the same rate. HOwever, there would still be a concetration gradient in the air, due to the conditions before the elevator's release. Thus, we would expect the balloon to "float up" initially.

 

HOwever, quite quickly the gas would diffuse from the area of greater concetration to the areas of less concetration, untill the air in the elevator was of uniform concetration. This diffusion would be in a net direction ("up"). Thus there would be a slight wind, do to the moving air. This wind would carry the balloon up as well.

 

Once the air's concentration was equilibriated, there would be no boyant force acting on the balloon. Thus there would be nothing pushing it "up". There would be no other net force (from the frame of the box) acting on the balloon either. Becuase of this, the balloon would continue to move up with its velocity gained in the two above paragraphs. However, eventually the balloon would hit the top of hte box and bounce "down". The balloon would continue to move about the box until the air resitance stopped it (this would take a while). At this time, the balloon would be falling at the same rate as the box and hte air in the box, and would not be observed to move.

 

The point being that;

1) Everything falls at the same rate

2) After diffusion of the air in the box, there will be no net boyant force on the balloon. Thus, it will not float "up."

3) Once everything stops moving from its initial perturbance, things will be stationary with respect to eachother, due to point 1.

 

Of course this is only for the case of free fall, wich was the original question. It is one of the cool results of a uniform gravitational feild (wich is basically just something that will ensure everything, all the molecules and everything, will be accelerating at the same speed).

 

 

Anywyas, that is how i think it would happen. Sorry about that original post, i gotta learn to think things through before i write *sigh*

[YT2095]

and of course taking into account that Helium is considerably lighter than "AIR" even with that balloons weigh in rubber taken into account, bouancy will have an effect

[VendingMenace]

but in free fall, there will not be (cannot be) a net boyant force.

 

Boyanc force = (volume of substance displaced)*(density of displaced substance)*(net gravitational acceleration)

 

so, in free fall the net gravitational accleration (from the frame of the thing falling) is zero, and so the boyant force must also be zero :/

[Kedas]

anyone willing to do a guess/calculation on the speed of the balloon before all forces are zero?

Very small if you ask me.I think using the word 'wind' is an overstatement here.

[VendingMenace]

yeah, well of course, but what else do you call a moving mass of air? Breeze? Anyways, wind is not very quantitative, but i think you get the point.

 

As far as calulation goes, we would need to know what the starting heighth was for the elevator, the mass of the balloon, the pressure of the helium in the balloon, the rpessure of the air in the elevator, how tall the elevator was, the location of the balloon within the elevator, the viscocity of the air, and the resistance of the air.

 

But other than that, it should be pretty straight forward. Espcailly if you make some simplifying assumptions. Perhaps if i am bored this weekend i will do it. But don't count on be being that bored

[Kedas]

Let's make it simple first what are the pressures in function of time of a air column after the release of gravity (or free-fall)

 

ones we have that 'we' can calculate the pressure on the balloon over time.

The pressure ballancing will go as fast as the speed of sound I guess.

 

Your allowed to be a factor of 5 wrong I just want ot have an idea.

[YT2095]

In free fall (or zero G) , the balloon would "seek" the area of lowest air density, if this was not obtainable, it would remain in situ.

[Atlantic]

to sum things up, we have no gravity.

According to the familiar principle of the Greek mathematician Archimedes the buoyant force is equal to the weight of the liquid that is displaced by the object. Since there is no weight, there is no buoyancy.

If there is a no-gravity environment in the elevator the perosn standing there would have 0 pressure on the feet. Therefore, there will be no air pressure or concentrations, as there are in the atmosphere.

Therefore, the balloon will stay in the middle.

Concluded for now................. now...........

or.........We excluded the point that the elevator falls faster than air. We shall forget gallileo because we are talking about different state mediums. So will there be air compression spots or will there be equal pressure distribution throughout the elevator. since there is no other force exerted on the balloon i think it will act as the air translates position. But the question remains.

I am guessing "Wind" is referred to the vertical translation of air particles inside the elevator creating different concentrations. So when the air particles move to the top of the elevator, they will cary the balloon with them, based on air resistance. And then if the acceleration remains uniform, it may move down. The acceleration has to be uniform for the constant upward force to be applied on the air. The force is also exerted on the balloon.

I am right or am i wrong?

[VendingMenace]

i don't understand you summary, but i would like to point this out...

 

We shall forget gallileo because we are talking about different state mediums

 

Huh? In a gravitational feild ALL things fall at the same rate. This does not matter if it is air, water, a hammer, a feather, whatevere. They all fall at the same rate. Of course, we must assume that the box itself does not expreience any drag, but i think that is a fair simplifying assumption to make, esp since the box could be falling toward the moon or mars where there is little or no atmosphere, and hence little drag.

[YT2095]

a lift (any lift) isn`t actively PULLED down, like Vending said, Gravity does all the work there

ok a feather MAY fall slower than a house brick, but that would ONLY be due to resistance of gas molecules colliding (Air in this case).

try to think of it this way, Imagine you`re in orbit in the space station, and you release a heleum balloon, it`s lighter than air sure! but it will not float upwards

it only does that here because the air is denser the closer you are to Earth, than it is higher up, and so it naturaly it wants to reach it`s own equilibrium of gas density over molecular weight.

in the space station, it would be treated as it were just an air molecule, but it WOULD actively seek out areas of lower air density