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You are in a car going 60mph with a balloon in the car. You slam on the gas, which way does the balloon go? (No really, get a balloon and do it, it will freak
by ransom1538 8y ago
You are in a car going 60mph with a balloon in the car. You slam on the gas, which way does the balloon go? (No really, get a balloon and do it, it will freak out everyone in the car).
- duncan-donuts 8y agoIs the balloon filled with helium gas or air?
- comboy 8y agohelium solution, for those busy or lazy: https://www.youtube.com/watch?v=y8mzDvpKzfY https://www.youtube.com/watch?v=y8mzDvpKzfY
- Pharmakon 8y agoAssuming a helium balloon, it moves back toward the rear.
- Pfhreak 8y agoIt does the opposite, in fact. When you accelerate, the gas in your car behaves like a fluid, piling up in the rear of the vehicle. Helium balloons climb up the density gradient of air, essentially seeking the lowest density point. That area of low density is at the front of the car as you accelerate. So, the balloon moves forward as you accelerate.
- chairmanwow 8y agoIs the gas density gradient really so significant that it allows a balloon to move towards the front of the car? Any way we can add some calculations to this?
- FakeComments 8y agoSmarterEveryDay does a video on this — nothing like experimental data. https://www.youtube.com/watch?v=y8mzDvpKzfY https://www.youtube.com/watch?v=y8mzDvpKzfY
- bzbarsky 8y agoThe density gradient is not really that important. You could do this with a car filled with water and a cork instead of a balloon, and the effect would be the same, even though the density gradient in water would be pretty darn close to zero. What matters is that the direction of the buoyant force is precisely opposite the direction of the overall force acting on the fluid (because that's what keeps the hypothetical fluid that your balloon displaces from moving due to said overall force). Importantly, this overall force needs to be measured in the reference frame of the fluid, which in this case is accelerating. OK, so let's draw out a force diagram on the air inside the car. There's a force upward from the floor, exactly balanced by the force of gravity down. There's similarly a force from the back of the car , which is what's accelerating the air, from the point of view of a non-accelerating observer. From the point of view of an observer in the car, however, the air is _not_ accelerating, but the force from the back of the car is still there. So there must be a force backwards on the air, to balance the force of the back of the car. You can call it "inertial force" or "gravity" (in the general-relativistic sense) but the upshot is the same: the overall force on the air is down and backwards, so the buoyant force is up and forward, along the same line. For a helium balloon the buoyant force is stronger than "gravity", so it goes up and forward.
- Pharmakon 8y agoTragically I seem to have read “slam” and my brain filled in “brakes”. I only recognized my error when you commented. Not my proudest moment.
- gameshot911 8y agoI made the same mistake! Truly suprised to go back and see that it said 'gas', would have sworn it said brakes.
- Sharlin 8y agoIt’s the brain’s autocorrect in action. At 60mph slamming the brakes makes more sense than the gas. I presume GP actually meant brakes.
- jimmaswell 8y agoIn either braking or accelerating, is there a chance the balloon gets caught in an eddy and spins around a bit as the air pressure redistributes itself and goes around seats/heads/etc, or would the eddys be negligible to the balloon's mass?
- Simon_says 8y agoIt stays where it is, held tightly in my lap, because I know a loose balloon in a vehicle is a hazard.
- Stratoscope 8y agoI am sorry you were downvoted for that. Let me just go on record to say I got a smile out of your comment.
- Simon_says 8y agoNah. No big deal. Karma is irrelevant. I make a new account every few months anyways to avoid doxxing. I just say what's on my mind and people can appreciate it or not.
- SmellyGeekBoy 8y agoYou've been doxxed on HN!?
- Simon_says 8y agoNo, and I'd like to keep it that way.
- Diggsey 8y agoSlamming on the gas at 60mph probably will freak out everyone in the car, but not because of the balloon :) I'm assuming you meant slam on the brake, in which case the answer below is correct.
- Wowfunhappy 8y agoActually, slamming on the break at 60mph will probably freak everyone out too, due to the jolt, albeit less so than slamming on the gas.
- King-Aaron 8y ago> albeit less so than slamming on the gas I dunno, my turbo miata will light up the rear tires at 60mph if you drop a gear and stomp on the throttle
- SmellyGeekBoy 8y agoCool story bro
- jimmaswell 8y agoIt's been established that it moves in the direction of acceleration, but an addendum I thought of is that the balloon's speed will at first keep getting faster as the gradient is built up as the air is squished to one side, then slow down as the air pressure reaches equilibrium again.