Suppose you’re driving in your car, and there’s a helium balloon floating at rest over the passenger’s seat. You suddenly hit the brakes and the car decelerates. We all know that any loose items on the seats or dashboard will slide forward as their momentum carries them “ahead” of the car.
What does the helium balloon do?
The balloon scoots backwards due to air pressure changes.
How is slamming on the brakes in a closed car (necessary for the balloon to be "at rest) going to change the air pressure inside the vehicle?
Imagine the car is partially filled with water and an object is floating on the water. What happens to the water and the object when the car stops?
The only time the balloon moves is if the air moves. While riding in a car with the windows up (not including air movement from ventilation system) the balloon remains still becaise the air in the car is not in motion despite the car moving down the highway.
I believe Blake has it right. The deceleration will cause a vsery slight pressure gradient in the air, with a slightly higher pressure at the front than the back. The baloon will move toward the lower p[ressure at the back.
rowrrbazzle has basically answered this. The car decelarates but the air itself continues forwards just as any other fluid would. As a result you get a wave of high pressure at the front of the vehicle causing the balloon to move backwards.
Wrong. With the windows up the air sloshes around like, well like a fluid in a closed vessel. Flickster have you ever tried this simple experiment? If you had you would notice that the balloon moved forwards when accelerating, backwards when decelerating and moves to the inside on corners, precisely the opposite of the way solid objects move.
My initial thought is that the balloon’s forward momentum will first make it want to lurch forward, and THEN start moving to the back due to the air pressure…
Why ask people here what’s going to happen? Why guess? Buy a helium balloon for a buck (or go to a restaurant that gives 'em free to the kiddies) and try it yourself. Nothing quite substitutes for hands-on experimentation.
If you’ll be watching it too closely, though, have someone else drive.
You’ll really like watching the balloon when the car corners hard.
Do only less-dense-than-air objects behave this way, or would the pressure difference affect any light objects in the car? If you hung a sheet of paper from the ceiling on a string, would it also move back when you decelerated?
I would try this myself, but I don’t have a car.
I fyou do do the experiment report back with your findings.
It probably does, but it’s not really noticeable. The balloon is light, so the inertia is slight and that slight inertia has to overcome the resistance of the air to move forwards so any forward motion is highly restricted. It can however move backwards because it’s being moved by air pressure.
It only works for lighter than air objects. That’s because only lighter than air objects are have allow enough density to be pushed around by air pressure, that’s why the float upwards. More dense objects like paper can’t be moved by normal air pressure. Remember we are talking incredibly minute changes in pressure here.
I have kids that take balloons home from birthday parties and I have done this many times. They float towards the back. And not surprisingly, when you turn, they drift towards the inside of the turn instead of the outside for the same reason.
Cecil Adams on Is air subject to inertia?
Another way to think about this is that the direction of “up” changed while you are accelerating or decelerating. According to relativity, acceleration is indistinguishable from gravity. So when you step on the brakes, the deceleration adds to the gravity. The combined effective gravity no longer points straight towards the ground, but slightly towards the front of the car. So if you have a pendulum in the car, it will point towards the front of the car. If you have a helium balloon on a string, it will point towards the back of the car.
As far as I know, Newton’s laws of motion have yet to be repealed. The First Law: An object in motion continues in motion until acted upon by an outside force.
Initially, the balloon is at motion with respect to the outside environment in the same way and with the same velocity vector as the car. (Relative to each other, they are at rest.) When the car decelerates, the balloon continues in the same direction as before until it hits something. Therefore, it would move forward relative to the car, exactly as a human body or your coffee cup does when you hit the brakes. Why should a balloon be different?
Air in the car would move forward, too. The only difference is it has very little mass, and it hits the windshield pretty quick since it is already there. Any pressure gradients, unless you are talking about near-speed of light decelerating to near-zero in a very short period of time, will be insignificant unless the airbag goes off.
The contents of the balloon, assuming a gas, are also insignificant. They may cause it to stay near the top of the car (helium) or the bottom (any heavy gas), but that’s pretty much it. They should have very little effect on the forward motion.
Would the baloon not be in contact with the roof of the car?
If so wouldnt it initially move forward like other object, and then in a way "bounce off " the roof, therefore pushing it back.
I agree that the movement of air has something to do with this, but I am wondering what the effect of the contact with a fixed point of the car may have on the balloons movements.
Also to test this you would need someway of seeing the air inside the car move. So smoke may be needed which may make driving unsafe
Which is why the air rushes towards the front of the car when you brake. The balloon also wants to do the same, but is pushed aside by air because a helium balloon is lighter than air.
Totally unrelated but slightly amusing anecdote involving a helium ballon:
Last year I was given a mylar helium ballon at my birthday party. It floated in the corner of the living room for a few months; as the helium sloooowly escaped, it settled lower and lower until it finally rested on the carpet, drooping and sad-looking like a post-coital condom-encased p… well anyways, it wasn’t quite so perky anymore.
My wife said, “Are you going to throw that balloon away?”
I replied, “Yeah, of course!”
A week or so went by, and I didn’t throw the balloon away. I was busy watching TV and stuff.
I returned home one eve, tired after a hard day of… erm… whatever it was I was doing that day (unemployed at the time). Astrogirl was already home, and puttering about. I sat on the couch with a cool drink, and turned on the TV.
Suddenly, Astrogirl audibly gasped, and eyes wide, pointed into the corner of the room. “Look!”
I looked.
And I looked again, brain sputtering.
The balloon, which had of late been fluttering about the floor over in the corner, was now, despite being emaciated and devoid of bouyancy, pressed firmly against the ceiling! :eek:
“How it can do that??” my wife asked in English broken by her puzzled state, “It was EMPTY!”
I babbled. I blustered. I looked at my wife, and the confused yet trusting gleam in her eye challenged me to provide an explanation, and be quick about it!
Think, Mr. Science-explains-everything guy! C’mon, Logic Boy… Mr. Know-it-all!! There HAD to be some scientific explanation for this oddity… not something obvious, obviously, but rather some esoteric atmospheric anomaly. I dared not ponder a Nobel Prize for discovering the explanation; perhaps a cover story in Scientific American…
THINK dammit!
“Ba, I… duh… maybe the air pressu… no… the temperature! No. Ummmmmmm… It, when… the buh… errrrrr…” I stuttered.
At this point, my wife collapsed to the floor, holding her cute little belly in laughter.
It was only a few seconds later that I discovered the scotch tape holding the balloon to the ceiling.
Note to self: Wife is smarter than me. And evil! Take heed…