Law Breaking Balloons

Podcast

Law Breaking Balloons

Transcript

Why balloons in cars are so contrary. I’m Bob Hirshon and this is Science Update.

Today's question comes from Dennis James, Of Phoenix, Arizona, who's been driving kids to and from birthday parties.

James:
Why does a helium balloon act the opposite of Newton’s law? That is, if you put a helium balloon tethered by a string in a van, and the van accelerates, the balloon goes forward. If the van decelerates, the balloon goes back. It does just the opposite of what a passenger would do in a van.

We asked the aptly named Vann Priest, a physicist at Whittier College. He says Newton's law is working just fine. That's the law that says objects in motion tend to stay in motion.

Priest:
This is why, when the driver applies the brakes, people lurch forward.// The balloon, you would think, would do the same thing. However, what we often forget is that the car is also full of air.

And when the driver puts on the brakes, the air lurches forward too.

The balloon behaves differently because it's lighter than air. So when all that air gets thrown up to the front, the balloon gets forced toward the back, where the air is thinner. And when the van speeds up again, and pushes the air and the passengers toward the back of the van, the balloon actually gets nudged forward.

If you've got a science question, call us at 1-800-WHY-ISIT. If we use it on the show you'll get a free Science Update mug. For the American Association for the Advancement of Science, I'm Bob Hirshon.

Making Sense of the Research

The scientific concept of motion is something that is often misunderstood. Many people believe that constant speed needs some cause to sustain it. Many also believe that the amount of motion is proportional to the amount of force, and that if an object is moving there is a force acting on it in the direction of the motion. These misconceptions may be difficult to overcome. It's important to have ample opportunity to observe objects in motion and discuss what happens to them. Balloons, in particular, can be confusing because even when we understand Newton’s laws, they seem to act contrary to what we believe should happen.

The Science Update you just heard tackled the case of helium balloons in a moving vehicle. It did a good job of showing that the balloons are indeed following Newton’s law, and not acting contrary to it.

Now try to answer the following questions:

1. What happens to a helium balloon tethered by a string in a van when the van accelerates and decelerates?
2. What does Newton’s law say about objects in motion?
3. How do gravitational fields affect time?
4. When the driver of a car puts on the brakes, what happens to the air in a car?
5. Why does the helium balloon behave differently than the air?

Going Further

For a thorough explanation of motion, go to The Physics Classroom’s presentation of Newton’s Laws of Motion. A particularly interesting part of this presentation is a GIF animation called The Car and The Wall. This animation explains and demonstrates what happens to a person in a car when a car crashes into a wall.

Another interesting look at Newton’s laws is a ThinkQuest site called The Physics of Baseball. This site takes a look at Newton’s three laws of motion and the law of gravity.