Resource: Balloon Brain: Designing a Helmet

Undoubtedly, one of the most important organs in your body is your brain. It's no accident that adaptations through evolution have completely enclosed our brains in the protective structure of our skull. Thanks to our skulls, falling in the bathtub or bumping into the doorframe is rarely fatal or debilitating -- aside from those moments of extreme pain shortly after impact. And while the skull is tough, it unfortunately doesn't always provide adequate protection for our brains. This is why it's important to always wear a helmet during high-speed activities like bicycling, skating, and skiing, activities in which falling -- and falling fast and hard -- is a very real possibility.

Helmets provide an extra layer of material between our heads and whatever we might crash into. But helmets aren't made of just any material or in any shape that happens to look nice. A pillow tied around your head would provide some protection against small bumps, but it would be nearly useless in the event of a high-speed bike crash. Helmets are designed to provide a much higher level of protection in the event of such violent crashes.

When you bump against anything, your body undergoes a sudden change in speed, from high speed on impact to a complete halt. In lay terms, we say that your body decelerated, or stopped, but scientists use the term acceleration to refer to any change in speed. Helmets protect by extending the period of time it takes for your head to accelerate. This acceleration may take only 1/100 of a second in an impact without a helmet. A helmet, however, extends this time to 7/100 or 8/100 of a second. By lengthening the period of acceleration, the helmet reduces the force applied to your head at any given moment. A rigid and well-fitting helmet also distributes the force around the helmet structure so that less of the total force focuses on the point of impact. Lastly, the slick surface of a helmet slides more easily across the road or ground than a bare head does, further lengthening the period of acceleration. These features may not sound like much, but they can make the difference between life and death or brain injury.

Bubble wrap and egg cartons are two of the materials used to solve the problem of protecting the water balloons. Describe the properties of each material that make it a good choice. Are there any downsides to using either one? What other materials would you use? Why?

Did you notice how the water balloons came to a complete stop very suddenly when they hit the wall and then the floor? How did the homemade helmets protect the balloons at the moment of impact?

Take a look at some bike helmets. In addition to the materials that make up a bike helmet, what other criteria do you think designers take into consideration?