4 ms·

Assuming a helium balloon, it moves back toward the rear.

Pfhreak 8y ago
It 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 ago
  Is 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 ago
    SmarterEveryDay 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 ago
    The 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 ago
  Tragically 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 ago
    I made the same mistake! Truly suprised to go back and see that it said 'gas', would have sworn it said brakes.
    - Sharlin 8y ago
      It’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 ago
In 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?

It 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 ago
Is 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 ago
  SmarterEveryDay 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 ago
  The 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 ago
Tragically 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 ago
  I made the same mistake! Truly suprised to go back and see that it said 'gas', would have sworn it said brakes.
  - Sharlin 8y ago
    It’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 ago

In 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?