Mardi Gras means partying, parades, king cake and of course beads. Lots of beads. And if you have kids the beads are probably their favorite part. Instead of tossing them after the kids have lost interest, try this simple (and addictive) demonstration.
Inertia beads first went viral in 2013 with this BBC video . However, the explanation of why the beads pour out of the jar – once they start moving you CANNOT stop them, believe me I’ve tried – and create the large arch was not quite right. In 2014 a group of scientists explored the question a little deeper, created some models and here is what they found.
If you use a string of beads that are spaced out just a little bit further they still pour of the jar. This supports the original theory that inertia (an object in motion stays in motion unless a force changes its motion) keeps the beads moving. However, the spaced out beads do NOT make an arch over the edge of the jar. This is the part the original theory got wrong.
The string of beads doesn’t behave like individual beads attached by string. Instead they act like rods connected by string where 3 beads make a rod. As one end of the rod/beads are pulled out of the jar the rod is lifted but also turns from flat to vertical. In the process the bottom end of the rod/beads pushes down on the rest of the beads on the jar. Those beads in turn push up on the rod/bead (as stated in Newton’s 3rd Law every force has an equal and opposite re-force) giving it a little upward kick. All of those upward kicks add up causing the string of beads to go up in an arch before falling down to the ground.
For more on the science of Inertia beads see this nice article with videos from Nature.
Make your own Inertia Beads
Cut each string of beads with scissors and use the glue gun to glue them end to end to make one large chain.
Find a jar or vase with straight sides that holds all of your beads. Start at one end and feed the beads into the jar so that there are no tangles or knots. When you are ready, pull the end of the beads out and just let go!
- If the beads starting flowing out of the jar before you are ready put your hand IN the jar. Grabbing the beads already out of the jar does not stop the flow.
- For a bigger arch hold the jar higher. In the video we stood on the archway at the top of the stairs at my kid’s school so the fall was a good 12 feet. Holding the beads higher gives them more gravitational potential energy so that when they come out they have further to fall. The further the fall, the faster they move and the bigger kick your twisting rod/beads get giving a higher the arch at the top.
Keep it going
Make Inertia Beads into a science fair project! Here are some questions to investigate.
- How does the shape and size of the container affect the speed of the beads and the height of the arch? Tape a ruler the wall and use a cell phone or tablet to record the beads to measure the height of the arch. Measure how high the jar is from the ground and time how long it takes the first beads to hit the ground.
- What if you line the jar with felt or some other soft material that absorbs forces? Does this affect the height of the arch?
- How does the size and shape of the beads affect the speed of the beads and heigh of the arch? Look for beads that are smaller (check the hardward store) or larger than mardi gras beads?
- Do you really need a jar? Try laying the beads out on a table top and dropping the end off the edge. Do the beads still make an arch?