eli5: Motion of a helium balloon in a car

Question

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.

In: 8

Answer 1

Think of it as a seesaw. For something heavy to move backwards in the car, the lighter end of the seesaw has to move forward. Air is denser than helium, so the air inside the car is the heavy end of the seesaw and the balloon is the light end.