1. Tablecloth Trick
I wrote this science experiment on January 1, 1998, when my Experiment of the Week list was just getting started. This was long before I started using Yahoo Groups or Topica, so after writing each science experiment, I would paste in the addresses, 20 at a time. When I wrote this one, I was spending about 4 hours to send each week's science activity.
Even now, ten years and hundreds of weekly experiments later, this is still my favorite way to demonstrate the science of inertia. The demonstration is dramatic enough to make people tense in anticipation, even if they understand the science and have seen it done a hundred times. The science is simple enough for young students to understand, and with practice, even a first grader can successfully perform the demonstration.
You will need:
- a piece of cloth or sheet of newspaper
- two heavy coffee cups (cheap ones in case you break them)
- two saucers (the same thing goes for these)
- a smooth table top
If you are using a piece of cloth, be sure that it is smooth and that the edges have not been hemmed or sewn. You don't want anything that could catch on the bottom of the dishes. If you don't have a piece of cloth available, try using a sheet of newspaper. Be sure to have several sheets available, because they tear after one or two tries, and you will probably want to try this several times.
Lay the cloth or paper on the table, with about half of it hanging off the edge. Place one of the cups on the cloth, about six inches from the far edge. Grasp the hanging end of the cloth firmly. Now comes the part where you have to trust me. If you pull on the cloth gently, the cup will move with it and fall off the table. To get the cup to stay in place, you must jerk the cloth very quickly, downwards and away from the table. Use one quick jerk to pull the cloth completely out from under the cup. If you do it quickly enough, the cup will hardly move.
Understanding the Science
Why did it work? The cup stays in place because of inertia. Inertia causes things that are sitting still to continue sitting still until something pushes or pulls on them with enough force to make them move. The heavier an object is, the more push or pull it takes to overcome their inertia and get them to move.
If you pull the cloth slowly, friction between the cup and the cloth transfers enough pull to move the cup. If you pull the cloth quickly, the cup's inertia causes it to slide on the cloth, letting the cup sit still as the cloth is pulled away.
Now, lets make it more exciting. Put the cloth back on the table and place a saucer where you put the cup earlier. Place a cup on the saucer. Stack the other saucer on top of the cup, and put the other cup on top of that. Now you should have a tower, with a cup sitting on a saucer, sitting on another cup, sitting on another saucer, sitting on the cloth.
Do you think you can pull the cloth out from under it now? Try it, again using a very quick pull. You will find that it is even easier than with the single cup. The more weight you have piled up, the more you have to push or pull to overcome inertia. That is why light, plastic cups don't work well, unless you put something heavy in them. To really impress your friends, use a glass filled with water. The water makes the glass heavier, making the experiment easier. Just be sure to practice a few times someplace where it will be easy to clean up the mess, just in case.
Have a wonder-filled week!
