Space and Physics
If you've ever transported helium balloons in your car, perhaps courtesy of a small child who won't let you store them in the trunk, you may have seen something odd.
When you accelerate in a car, the seat you are sat in pushes you forwards, with the result that you feel like you are being pushed backwards into it. If you dangle something on the rearview mirror, you will also notice that it will swing backwards as you begin to accelerate.
So what happens to a helium balloon, flying freely inside your vehicle? Surely it too will be thrown backwards as you accelerate in the forwards direction?
Well, as you may have already intuited, no. That is not the case. As you accelerate forward, the helium balloon will be thrown forwards too. It's a strange sight, especially when compared to a balloon filled with typical air rather than helium.
Conversely, when you brake the helium balloon will be propelled backwards, while an ordinary balloon (and you) are propelled in the forward direction.
So, why is this? It is because of the density of the gas inside and outside of the balloon.
Buoyancy is an upward force in a fluid (any flowing substance, including air) exerted on all bodies within it. The force comes from the pressure within the fluid being greater the further down the fluid you go. The pressure on the bottom of an object within the fluid is higher than at its top, causing the upwards force.
If the buoyant force of a fluid is greater than the weight of an object placed within it, the object will float. Helium, being lighter than the other elements in our atmosphere, rises. It's the same when air is heated inside a hot air balloon, making it less dense per volume inside the balloon than it is outside, causing it to rise.
So, when you accelerate your car forwards, the air within your vehicle sloshes backwards, like if you were to accelerate a bucket or a jar of water.
This air, ever so slightly pushed towards the back of the vehicle, creates a buoyant force now pushing the helium balloon towards the front of the vehicle. When you brake, the air moves towards the front of the vehicle, and the buoyancy force pushes the balloon backwards, resulting in a weird-looking effect.
