Question

A little girl sits in a car at a traffic light holding a helium filled balloon. The windows are closed and the car is relatively airtight. When the light turns green and the car accelerates forward, her head pitches backward but the balloon pitches forward. Explain why.

Answer 1

The air in the car has inertia. It resists changes in its motion. So do the balloon and the girls head.
When the car accelerates, all three will resist the change in the motion. The girls head and air move to the back of the car (well, some of it, not all of it). The balloon would too, However, because the air has moved to the back of the car, there is a greater pressure in the back. Things that are not very dense (like helium) are very sensitive to that change in pressure, so it will feel a force (caused by the difference in pressure) and be pushed forward. The girls head also feels this force, but it is much more dense, so it is not as sensitive to the change in pressure.