The Girl with a Helium Balloon
Sara is returning home from a birthday party in a van. She has a helium balloon tied to the seat in front of her. If the van driver suddenly steps on the brake pedal very hard, what will happen to Sara and her balloon, relative to the van?
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10 solutions
How do you know that air molecules have forward motion?
This is not clear for me as air particles moves in random motion.
What if we create vaccum inside the bus ?
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You don't feel any wind inside the van (assuming windows are up, no A/C, etc). That means the air molecules, on average, are at rest relative to the van, i.e. they are moving forward together with it.
If we create a vacuum in the bus then the balloon will also lunge forward provided it doesn't burst. This is because the surrounding air is absent and no force acts on the balloon and it continues to move forward while the bus stops.
On the other hand, if the bus has air, then the air being heavier than the helium gas inside the balloon has a greater tendency to lunge forward and push the balloon backward. This is similar to why a helium balloon flies upwards despite the gravity acting on it in the downward direction.
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Thank you Sir , as second one was my curiosity and first one interested the use helium in it . :)
I may be wrong but I think a lot of people got this answer correct by making false assumptions and failing to take air pressure into consideration. I don't think that 50% of people on this site have either enough knowledge of fluid mechanics or the mental capability to intuitively come to the conclusion that a sudden decrease in velocity in an enclosed vehicle will increase the air pressure in the front of the vehicle and that consequentially the balloon will travel to the space of lower pressure. It doesn't seem too complicated written out but without much background knowledge I think it would take a decent amount of intuitive leaping.
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Or maybe they have been in cars with balloons. My parents did a lot with empiric knowledge
Is the argument that the child moves forwards and therefore air is displaced by her which has to travel backwards to fill the void behind her - and this exceeds the forward motion caused by the balloon's inertia?
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Even if the child is not in the van, the same effect with the balloon will be observed. When the van brakes, there is an additional 'apparent' gravity forward, and the balloon being less denser than air, moves against gravity, which is backward. We have the implicit assumption that the mass of the balloon is small enough that the average density of the balloon is less that that of air.
I'm not buying it being that simple. If the air moves forward, then the balloon would experience a momentary dynamic pressure on the back side of the balloon, which along with the momentum of the balloon and the momentum of the gas inside the balloon will push the balloon forward. It would move forward, then back due to the slight difference in static pressure creating a buoyant force and horizontal component of tension on the string.
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It is very similar to why does a balloon fly against the gravity? The surrounding air being denser is pulled by the gravity stronger and hence wants to move down more than the balloon resulting in an upward push on the balloon.
Similarly, here the surrounding air being denser has a greater inertia and wants to lunge forward more than the balloon and hence result in a backward push on it.
"The air molecules will rush forward as the bis stops" Does that include the air molecules inside the balloon? implying it would move forward first, then backward?
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The balloon does not have air (nitrogen or oxygen) molecules, but helium molecules which are much lighter. In this case, the air moves forward around the balloon, causing the balloon to move behind.
Another way to look at this is that applying brakes causes an apparent gravity in the forward direction. Just like a helium balloon rises up against gravity due to buoyancy, the balloon moves back against the direction of the apparent gravity.
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The balloon is being pushed up due to buoyancy when you hold it on a string, it isn't directly due to gravity (but gravity does play a role). Hence why the balloon will move backwards in the car, the heavier air molecules have more forward momentum
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@Chris Cheek – Gravity does play a role in pushing the balloon up. The air outside the balloon is more dense than the air inside, so the helium balloon is pushed upwards. Without gravity, the helium balloon wouldn't rise up. Similarly in the car, the molecules outside the balloon have a greater forward momentum than the molecules in the balloon. So they move forward and the balloon moves backward.
Why would the air rushing forward not push the balloon forward? If you hold a helium balloon in the wind, does it move against the wind?
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If you hold a helium balloon in the wind, it does move with the wind because the wind applies a force along in its direction. In this case though, air moves forward around the balloon, causing the balloon to move behind.
Another way to look at this is that applying brakes causes an apparent gravity in the forward direction. Just like a helium balloon rises up against gravity due to buoyancy, the balloon moves back against the direction of the apparent gravity.
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No argue against that. Nevertheless I think the time scale matters. As the pressure in the front of the van rises the balloon should go forward by inertia. Once it feels the pressure gradient then it will move backwards. So I might guess the time scale of the first stage is too small to be experienced in the van case?
I just used prior knowledge knowing that when my dad breaks hard I usually lunge forward. I think guess and check works best. How does inertia work? Why does the balloon lunge backward? I would rate this problem a 10 because I don't know how inertia works. If I knew how it works I would rate it a 1. I didn't really get the word inertia. I was kind of confused.
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Inertia is the property of a body by which it resists changes to its state of rest or state of uniform motion along a straight line. The inertia of a body is measured in terms of its mass - more the mass, more the inertia. Think of a large rock for example. We will have to apply a large force to overcome its inertia of rest and get it moving (even if we neglect friction). Once it is in motion, we would again have to apply a large force if we have to stop its motion or change its velocity.
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You are right, the answer is wrong. It is misconception that the balloon bounces back initially everything will go instantaneously forward because of inertia at the braking. Then the air that went forward too will strike the windshield of the cabin and come back acting as a sound wave on the balloon. But the effect of inertia are instantaneous and the air travel with the speed of sound so it will take time to get back.
This is the answer but it's a bit vague.
I do dissent since the braking operation is sudden, the air in the van, the balloon and Sara all will go forward too initially, before the air strikes against the windshield and forward part of cabin. Then assuming windows are closed the air will return back and then pushing back the balloon. In Fluid Mechanics the analogy will be like an hydraulic ram, Now the pulsating wave of the air will make the balloon oscillate, frequency will depend on length of the van. Imagine the forward part of the cabin is far away it will take a while to the air come back.
i thought the balloon wont move at all , because the is no force that moves that balloon
https://youtu.be/y8mzDvpKzfY Just watch the video in the given link. The answer to all your questions.
While the bus is moving, Sara will have inertia force to move forward (same direction with bus), so when the bus suddenly stops, this inertia causes her to continue her original force and so will lunge forward.
However, this effect is contrasted to the Helium balloon, which is lighter than normal air. To elaborate, when at rest, the buoyant force causes the balloon to go up to the sky, the opposite direction of the gravity acting upon Sara. Then coming back to the bus halt situation, the inertia and gravity causes Sara to stay down and go forward, so the opposite forces (back- and up-ward) will affect those objects lighter than normal air, the balloon in this case.
Buyont force acts perpendicularly while the inertial force acts horizontally. Please have a look at my answer.
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Buoyant force acts along the effective gravity. When the bus brakes, there is a horizontal component in the effective gravity. The net effective gravity is g 2 + a 2 . Its horizontal component is a forward. Buoyant force acts against gravity since helium is lighter than air, so the balloon is pushed backward.
Terry's right. A low weight does not cause the change in the non-gravitational force you describe.
Ya, but relative to van doesn't it feel like Sara goes backward and ballon forward. Just think we talking about respect to van not to repeat with ground.
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When you are sitting in a van and brakes are applied, you do fall forward relative to your seat. This is why drivers hit their head with the steering wheel when they brake too fast and are not wearing seat belts.
This is complete nonsense.
You haven't elaborated why the balloon would move backwards. It moves up because of the buoyant force, opposite to gravity, true. But you are merely suggesting it is somehow predisposed to oppose all forces with no reason at all, in this case to inertia.
The specific weight of Helium does not effect, since the balloon is an enclosed system, and tension of string will take care its positive buoyancy. The core of the problem is in my opinion the wrong assumption that build up of the contra-pressure is instantaneous, what is not. When the air start to move forward it will keep moving forward till kinetic energy of air mass behind balance the work need to keep increasing of pressure due to compression, and its movement drag the balloon. As I said above the movement resemble to a hydraulic ram or if you prefer it like an spring on which you release suddenly a block of big mass, the spring will compress and its deformation it will be higher than the equivalent to the weight of block to later on bounce back an will keep its oscillation till, air resistance and inner friction damp the system. In the bounce back the balloon will move back again and then will keep oscillating with air. The mathematical analysis of the system is a cumbersone problem like most transient phenomena specially those dealing with not lumped masses.
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The exact analysis is cumbersome for any system. In this case we can see buoyant force acting due to effective gravity when the bus brakes. When the bus brakes, there is a horizontal component in the effective gravity. The net effective gravity is g 2 + a 2 . Its horizontal component is a forward. Buoyant force acts against gravity since helium is lighter than air, so the balloon is pushed backward.
What I got from Smarter Everyday is that the build up of air rushing to the front of the car and therefore the contra-pressure that builds up and pushes the balloon backward is instantaneous. I thought that the inertia pushing the air mass inside the car forward would momentarily push the balloon forward but this is clearly not the case.
Nice demonstration of this effect at Smarter Every Day some years ago.
Thanks for the info on Smarter Every Day.....
Tried it. Just like in the video BOTH WENT FORWARD. Brilliant is wrong.
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The balloon goes forward when the car ACCELERATES. The question is what happens when the car suddenly stops, and it's clear in the video that the balloon goes BACKWARD. Nothing wrong.
Thanks for the link!
Big thanks for sharing the link! Awesome stuff to learn:)
Good link. Actually explains it.
This was the first Smarter Every Day video I saw way back when.
I saw it too!!
Sara will move forward due to inertia.air will also move forward.therefore helium balloon will move backwards as helium is lighter than air
Nowhere does it say that the windows are rolled up. I assumed they were.
the rules/effects and moments of inertia acting on the girl but air pressure working on the balloon
Sarah has inertia and remains in Motion. The air inside the vehicle much like a tub of water moves forward as well, air the air being much denser than the balloon transfers the force on the airs momentum on the Ballon pushing to the back like it was stem on a winter breeze.
Inertia. Sara has more mass than air in front of her so she will have tendency to move forward, but the air around the balloon has mre mass than the balloon so it will continue "move forward" relative to the balloon... meaning the ballon will move back.
Sara lunging forward is apparent to anyone who has driven and accelerated/braked hard. The balloon I only knew instantly from Smarter Every Day: smarter every day balloon experiments
Sara is sitting sideways in the picture
After bus stop, Sara moving forward due to Inertia because Sara's density is much higher than air, but Helium gas' density is lesser than air so does the opposite effect.
Sara leans forward due to Inertia while when the van stops, air molecules also have a forward motion which was acquired when the van was moving. The air molecules will rush forward as the van stops. This increases air pressure on the driver's side of the van.
Hence the baloon moves backward.