Experiments With A Helium Balloon

Something on the science page of the newspaper caught my eye last weekend. A short article datelined The New York Times was trying to explain why a helium balloon inside a closed car would drift toward the direction in which a car is turning.

I thought about this for a long time. Common experience would tell us that an object floating free should be thrown toward the outside of a curve. The reason that a helium balloon is not, according to the Times release, was because helium, being less massive, accelerated (or decelerated) at a slower rate than the air around it. If you have a hard time following that, so did I. However, I felt strongly that any possible effect would be so small as to be unobservable in a car, so I went out and bought a helium balloon.

I tied the balloon down to the center of the car floor so that it was free to float in any direction, and drove off accelerating and braking and turning sharp corners. It was a humbling experience. It gives a positively eerie sensation to watch something behave in a manner which a lifetime of experience tells you is wrong. If you slam on the brakes, loose junk should pile up around your feet and on the dashboard at the front of the car. Instead, the balloon jerked toward the rear. If you accelerate, inertia tugs backward on your head, but the balloon seemed to be trying to beat the car to the next intersection.

I mentally apologized to the unnamed Times correspondent, although I still felt that the explanation could have been clearer. So let me try it my way.

Gravity and acceleration have exactly the same effect (in fact, they are identical, but let's not get into that). Assume you were standing in a rocket ship someplace in the galaxy, and that the rocket ship was accelerating at the rate of 32.2 feet per second. If your head was pointed in the direction of travel, you would not be able to tell the difference between the push on the bottom of your feet and the normal gravity you would feel on earth.

Sitting in a car on earth, you feel the acceleration due to gravity on the seat of your pants, except when you add an additional component of acceleration or deceleration by stepping on the throttle or the brake. Strictly speaking, there is no such thing as "deceleration"--it's just acceleration backward in the direction that you've already traveled. In a turn, you are accelerated sideways. When you combine the downward component of gravity with the sideways (or forward, or backward) component due to acceleration, the total "field" is briefly tilted, as the fluid level in a car partially filled with water would show you. A plumb bob would point perpendicular to the surface of the water, and when there is only air in the car, the air thinks that "down" is the direction in which the plumb bob points. The helium balloon thinks so, too, and like helium balloons should, it tries to move "up" against the air in this temporarily tilted artificial gravity field. One way to mentally picture what is happening is to think that the balloon is trying to move toward air of lesser density, and that when a car is accelerating (sideways, backward or forward), the air inside is briefly rarefied on the side (or end) toward the direction in which it is accelerating, and compacted against the opposite wall.

The behavior of the balloon reminded me of the flaw that is seen repeatedly in car chase scenes on TV. Have you ever noticed that whenever the chaser rams the chasee from behind, the head of the person in the leading car bobs forward (instead of the proper whiplash motion to the rear)? This makes one wonder if the heads of actors who participate in car chase scenes on TV are filled with helium.