Dr. V Show: Snap bracelet – Bi-stable structure

HONOLULU (HawaiiNewsNow)-Dr. V is back from Oceanit with another cool and simple science experiment where you use a tape measure, but make sure you get an adults permission before doing this "Weird Science."

Dr. V Show:  Snap bracelet – Bi-stable structure

Things you will need:

  • Old tape measure
  • Duct tape or electrical tape
  • Old scissors or snips

The Experiment:

Please do this and all other experiments with the supervision of an adult. Cut a length of tape measure about eight inches long. BE VERY CAREFUL– the cut edges will be sharp. Cover the tape edges immediately. Notice how rigid the tape is. What happens when you bend it one way or the other? Curve the tape around a small radius of about 1/3 of an inch so it starts to deform. You can do this by rolling it around a thin pencil, or rolling it with your hands until you start to feel it deforming (when you feel it pushing back, push harder). Try bending some tape with the inside of the curve on the outside. How does the tape behave now when you bend it?

How does it work?

Before you rolled the tape, the tape was only stable in one way, straight. If you try to bend it, it bounces right back to being straight. This is the state it prefers to be in and requires the least amount of energy. Measuring tape is made with a curve across the tape, which makes it much stiffer so that you are able to measure longer distances without it crumpling up. This is done by deforming the tape at the factory so that the bottom is just a little longer than the top, making the tape curve. When you deform the tape, you are doing something similar by bending it in another direction into a coil. When you roll the tape into a tight coil, the outer side becomes slightly longer than the inner side. Now your tape has two stable states. It will stay coiled, or will stay straight, making this a bi-stable structure. In between these two states, the tape has to become flat, which is a very unstable structure, so it doesn't stay flat very long. This is why snap bracelets will snap onto your wrist. When you slap your wrist, you force the bracelet into the flat state from which it quickly moves into the coiled state.

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