I heard that if a car accelerates from rest, a balloon would move forwards instead of backward. Why is this? Science says it’s because of density, but I don’t get their explanation. It also seems like it’s violating newton’s first law.
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When you hit the gas, everything tries to move backwards in the car. You do, the coffee in your mug does, the dice hanging from the mirror do. The air in the car, too.
Since the car is full of air, it can’t really move backwards at all – there’s just more air back there blocking it. What if there was *not* more air back there? What if instead there was something even lighter, which the air could push out of the way as it moved?
Well, a helium balloon is lighter than air, so as the air rushes back it forces the balloon forward.
This effect is exactly the same reason that the balloon floats up in the first place. The energy of the air falling to take its place is greater than the energy required to lift it.
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