Gas and Plasmas?

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 acclelerates foward, her head pitches backwards but the balloon pitches foward. Explain why

Answer 1

That's a manifestation of bouyancy force. When the car accelerations, the inertia of the driver's head and the air in the car forces them to stay stationary while the frame of the car moves forward. This effectively increases the density of the air toward the back of the car. The balloon has little inertia and is bouyant and therefore is pushed forward in the frame of the car.