Suppose you had the balloon filled with He and tied to the floor of your car, perhaps, around your hand brake and your car is at stopped. If you then 'hammer the gas pedal' you will be pushed back in to your seat whereas the balloon will go forward! Turn left, the balloon goes right. Turn right the balloon goes left, and stop - the balloon goes back.
It has to do with the motion of the air in your car. When you are at rest and then go forward all of the air rushes to the back of the car and you have created an area in the back of your car of higher air pressure. Since He is lighter than air it wants to 'float' away from that area and moves forward.
Answered by: Peter Feeney, B.S., HS Physics Teacher
When a car stops suddenly, the air inside the car continues to move forward a little longer than the car itself, due to the tendancy of an object to maintain its motion until a force acts on it (inertia). The air is stopped by the front of the car's interior and tends to 'pile up' there. As a result, the air in the front of the car becomes slightly more dense than the air in the rear of the car, creating more pressure on the front side of the balloon. This causes the balloon to move toward the rear of the car, where the air is less dense, just as the same balloon released outside will move upward, toward the less dense air.
Interestingly, a helium-filled balloon released on board an orbiting spacecraft will neither rise nor fall, since it is the weight of the air that creates the differences in air density here on the Earth. In free fall everything is falling at the same rate and is in a weightless state, so the air is of equal density everywhere on board the spacecraft.
Answered by: Andrew Franks, M.A., Physics Teacher, Lee High School, Midland, Texas
Your question, therefore, is equivalent to asking 'What happens to a helium balloon in a car parked facing down a steep hill?' The answer, of course, is that the balloon moves toward the rear of the car.
Answered by: Paul Walorski, B.A., Part Time Physics/Astronomy Instructor