Balloon phenominon: Inertia or Wind?

In the second half of todays column, The washday blues: Is it safe to wash clothing at the laundromat?, inertia is credited with driving a helium balloom to the rear of the vehicle when the brakes are stepped on. I don’t have any experience with the stopping portion, but in the taking off portion I have noticed helium balloons do come up into the driver’s seat prompting the usual yelling at the kids to keep them things under control. I had always thought this was due to wind coming in from the window, curling around the rear of the vehicle, and returning forward about the center of the car (mostly because this continues to happen while driving after inertia should have settled in). Does anyone else have any first hand experience with this phenominon? Is it wind or inertia driving the balloon forward? Has anyone experienced the backward motion when slamming on the brakes?

I’ve done this, and it works as you say: when you accelerate, the helium balloon moves forward; when you brake, it moves aft. Around a corner, it leans to the inside of the curve.

Wind has nothing to do with it. It’s simply the balloon seeking the least-dense air it can reach. That’s the same as saying that it’s due to the inertia of the air. You can think of the air as if it were a fluid, “sloshing” around inside the car in response to various accelerations.

Give it a try in a car whose windows are closed - the effect is very pronounced (and surprising to many people).

In case anyone’s interested, I offer a somewhat more detailed – if somewhat less authoritative – explanation of the balloon phenomenon at

And there’s a joke you might like at the end.

–Lowly guest JohnEGee

Unfortunately, I’ve a small quibble with the writeup touted above, but not with the air density thing. Near the end, when describing the big bang (theory), you say “everything is flying away from everything else through space.” This is incorrect and repeats a common misconception, i.e. that the big bang resembles a conventional explosion. It doesn’t. The big bang did not fling things out from a central point. If it had, the direction of travel could be traced back to a “center of the universe.” It can’t. There is no center of the universe per se, except that every single point in the universe is at the center, having initially been at the origin of the whole mess. (They used to call it a “singularity” but I beleive that term has fallen out of favor due to its insistence on “infinite” density and pressure).

In the big bang process, everything is moving away from everything else, but not because things are flying through space; the space between them is expanding.

At first something struck me as odd about your post. I searched around a bit and the best cite I could find explained it this way:

But still something bothers me. Even with the raisin analogy, the raisins started somewhere. No one is saying that they started at the same singular point, but measurements can be made from an outside observer determining absolute direction of expansion. Ok, so there is no way for us to become an outside observer to the universe. Still, in order for there to be a final product and an interim process, there has to be a beginning point somewhere.
When traveling in a car at say 60 mph, the car in front of us that is going 90 appears to be going away from us at 30 mph, the car behind us going 30 also appears to be going away from us at 30 mph. While car B & C seem to be moving away from car A, all three cars are traveling in the same direction. Is there no way to determine the direction we are traveling relative to the rest of the universe?
If not, and assuming that nothing is moving away from anything else but the space between everything is expanding, where is all of this space coming from? Is it some sort of cosmic flow or simply empty space coming into existance?

There has to be a point of origin, yes, but since it’s space itself expanding (not just the things in it), working backwards to that point of origin necessitates winding space itself backwards to a single point, so the point of origin is everywhere anyway.

As I understand it, this new space isn’t ‘coming from’ anywhere; it’s just stretching. (I’m not exactly sure what ingredients you’d need to ‘make’ empty space anyay).

Anyway, from our perspective, the furthest starts are moving away at a faster rate. When we have sufficient computing power, we might be able to draw up a model of how most of the visible galaxies are moving relative to each other and determine which is the best candidate for center “car”, as described above.

What value finding the center of the universe has escapes me, though, beyond deflating the ego of New York City when they find out it isn’t them.

I may have been unclear about the car analogy. The center car is meant to represent the observer, each car is moving away from the observer at the same speed and should continue to recede at an equal distance. The trailing car seems to be more distant because an identical signal takes longer to return from the trailing car than the leading car, but this is because the observer is traveling in the same overall direction as the other two cars so the rear signal has to travel past the point of origin to catch up to the observer and the front signal reaches the observer before it reaches the point of origin. In this example the speed and direction are known and thus the starting point (assuming all three vehicles started at the same place) can be deduced. If we knew where the “center” of the universe is then we could reverse the process to find our direction and speed. This information could be used to better chart the cosmos and knowing the overall direction and speed of travel of two objects may help facilitate an answer to realistic interstellar travel (a galaxy currently thought to be twenty light years away may only be two, or less than one). That, plus convincing New York that they are not the center of the universe, never pass up a chance to denigrate New Yorkers.

From the link to the result of the balloon experiment, a little further on:

Then it goes on to cite the following:

my bolding

Am I missing something obvious or do these contradict each other?

Greater density = faster speed of sound
Helium is less dense than Nitrogen.
Thus, the speed of sound is faster in Nitrogen (air).

And on a related note, why doesn’t the pitch of our voice change while we’re under water? (or if it does, I’ve never noticed it).

And, while we’re on the subject of the expanding universe, and since I can never seem to resist getting into these physics discussions, even though I’ll never fully be able to wrap my brain around all of it, but I always enjoy trying:

I do understand the raisin bread model, and how every point in the universe is indeed part of the exact same singularity of the past and thus can lay equal claim to being the “center”, BUT, there is a finite amount of mass in the universe, no? If that is the case, can we at least say from a given point that there is more mass towards, say my left then my right? And if that is so, can you be at a particular place in the universe where ALL existing mass is to one side of the observer? Is there an edge? And if so, can we not determine a center?

Do I have long way to go before I’ll get it?

Moe, I take back what I said about New York (except for parts of Manhattan :stuck_out_tongue: ). That’s my point exactly. Either there is a finite amount of “stuff” floating around in what appears to be a given direction, away from the original Plank event, forming both a center and an edge. Or, there is an infinite amount of stuff floating around, possibly in a common direction, and it’s coming from somewhere. Like Bryan Ekers said, someday we may have the computational abilities to figure it all out.

From the changing pitch point, I did a search and, WHOA :smack: , found an answer in the Straight Dope archives:

I also found more on the inertia of air mentioning the balloon experiment. Amazing what you find on this site :smiley: .

Any chance the movement of the balloon is due to the tilting of the car?

Maybe, a little, if the balloon was bumping against the ceiling of the car and riding the slant upward when the car leaned forward while braking hard. It’s an easy effect to control for; just tether the balloon to the floor of the car (say, to the small storage compartment between the front two seats, or to a brick on the floor in the rear passenger area) so it touches nothing but its own string.

You’ve just abstracted too far. Warm air is less dense than cool air because the molecules are (on average) farther apart. Helium is less dense than air because helium atoms are lighter than nitrogen & oxygen molecules. These two causes of density differences have very different effects on the macroscopic properties of the gas … At typical temperatures and pressures, a given volume of helium will contain about the same number of helium atoms as the same volume of air contains molecules of various gases; but those helium atoms mass a lot less and, having the same average kinetic energy as the air molecules at the same temperature, are moving a lot faster.