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.

First, let's recall some ideas that you may have:

1) The head of the girl pitches backward because of the inertia force, her body wants to be still, but the car moves, so the movement of her body is in the opposite direction of the movement of the car until her body is accelerated at the same rate that the car is.

2) The helium is less dense than the air, this is why the balloon floats.

Now, remember that the car is also filled with still air, this means that when the car moves, the air will also move backward as the head of the girl, and because the air is denser than the helium inside the balloon, the air will "displace" the air balloon in the direction opposite to its motion; if the air is moving backward, the air will impulse the ballon forward.

This is why the balloon pitches forward when the car accelerates.

Because of Newton's third law of motion, the little girl moves backward while the balloon moves forward due to the pressure difference between inside and outside of the car. As the car moves, the air also moves at a fast speed creating low pressure outside the car. Meanwhile, pressure inside the balloon is high, so it moves from high pressure to low pressure that is towards the front of the car.