Why do things made of rubber bounce? asks Khiana Lowe, a student at Long Island Lutheran Middle and High School in Brookville, NY.
If you’ve ever tried dribbling a wooden ball, or attempted to shoot a piece of string across the room, you know there’s something different about rubber. Rubber, whether natural or manmade, is very elastic. An elastic material is one that can be stretched to several times its own length without breaking – and then snap back to its original size.
Bouncy is just another word for highly elastic. And rubber is very, very bouncy. Natural rubber is made of long, flexible chains of carbon atoms, connected here and there to hydrogen atoms, coiled up into a tangled mass. In plastic, another material whose molecules are arranged in long chains, the chains are rigid. But rubber’s chains can twist and flex. When a piece of rubber stretches, the molecular chains uncoil; when the rubber is released, the chains retract, curling back into place.
Before rubber is turned into, say, rubber bands, sulfur atoms are added to allow the chains to form stronger cross-links. This process is called vulcanization. The late physicist Richard Feynman said that what goes on inside a rubber band is incredibly complicated. But Feynman suggested that we could get some idea by comparing the chains of molecules in a rubber band to strands of spaghetti.
Imagine, Feynman said, that your spaghetti has stuck together during cooking, strands glued here and there to other strands, making an annoying (rubbery) mess. Rubber has a similar structure, chains of rubber molecules stuck together here and there, the molecules on one chain cross-linked to molecules on other chains.
Why would this make rubber bouncy? Think about stretching and releasing a rubber band. When you stretch the band, some of the chains line up along the direction of the pull. And by stretching the chains, you have energized the rubber molecules. The increased jiggling of the molecules on the sides of the lengthwise chains, at the spots where they’re linked to other chains, act like hands tugging on a taut rope. So when you let go of a stretched-out rubber band, the chains kink up, and the band snaps back.
The extra energy you’ve given the molecules by stretching the bands also makes the rubber warmer. (You can test this by putting a rubber band against your lips, and then stretching it.) When the band snaps back, its chains relax, The molecules lose energy, and the temperature falls.
But if you keep the band stretched, the temperature will increase, and the chains will get tugged on even more by their frantically jiggling molecular links. As Feynman pointed out, that also means that a rubber band is stronger when it gets warmer, and so can do more work. (A rubber band, heated with a hair dryer, will lift a tiny attached weight, such as a paper clip.)
For more on bouncy rubber, download the pdf file “Why Does a Rubber Ball Bounce?’, at www.ias.ac.in/resonance/Apr1997/pdf/Apr1997p48-54.pdf.
