Elasticity and the Bungee Run

Imagine standing on a platform, preparing for a bungee jump. If all goes well, you'll start out in free fall. Then, the cord will stretch, slowing your fall until you stop and reverse direction. As the bungee cord shortens to its original length, it will pull you upward until gravity takes over, causing you to repeat the cycle.

Against the Grain -- The Bungee Run

The bungee cord's job is to stop your fall. It's a tough task, regardless of how much you weigh. Because of the pull of gravity, you accelerate during free fall at a rate of about 9.8 meters per second each second -- the longer you fall, the faster you move. In accordance with Newton's second law of motion, your acceleration teams up with your mass to cause you to exert a tremendous amount of force on the bungee cord. The cord has to deal with all that force without breaking.

A bungee cord also has to be elastic, so your stop at the bottom is springy instead of sudden, and so you get to repeat the trip a few times before coming to a complete stop. Elasticity is a substance's tendency to return to its original shape after being stretched. It's what makes a rubber band snap back to a small size after being wrapped around a thick stack of papers. It's also what makes a bungee cord pull a jumper back into the air at the bottom of the jump.

At the End of your Rope

Now, imagine trying to pull a bungee cord, one that's capable of stopping your fall and pulling you back into the air, as far as you can. You don't have gravitational acceleration to help you generate enough force to stretch the cord. All you have is the power of your own muscles and the friction between your feet and the ground to move you forward.

As you pull on the bungee cord, it gets longer. At the same time, it gets a little thinner. The change in length is proportional to the change in diameter -- the farther you pull, the thinner the cord gets. But the rubber in the bungee cord only compresses so much before it stops, preventing the cord from stretching any farther. But without the help of free fall, you won't get that far before you run out of steam.

The Pull of the Cord

While your bungee pull might seem easy at first, the farther you stretch the cord, the harder it becomes to take another step. This is because you're not the only one doing the pulling -- the bungee cord is also pulling against you. The cord acts like a giant spring. The farther you pull it, the more energy it stores to enable it to return to its original shape, and the harder it pulls on you.

Eventually, you'll reach the limit of your strength... and it'll probably be long before the bungee cord has stretched as far as it can. So if you really want to stretch a bungee cord to the limit -- and you don't mind the danger -- you're better off with the help of free fall and gravity.