Centrifugal Force

   2005-08-20      experiment

This is a great outdoor activity and was one of the class favorites of the year.

Preparation:
- one paper cup for each student (use a strong one, like those made for hot drinks
- Tie a string to the cup by making two hole near the top across from each other and create a loop. Make it just long enough so that if a child holds the string with their arm relaxed, the cup will not touch the ground.
- for each child: one penny, one small ball
- a large bucket of water

By now, we’ve learned about different kinds of forces forces. I asked the kids to name some (magnetism, gravity). I demonstrated gravity by dropping a penny in a cup. Gravity makes the penny stick to the bottom of the cup. If I turn the cup upside-down, the force of gravity makes the penny fall out. I told the class that I had a challenge for them. Did they think they could make it so when the cup is upside-down the penny didn’t fall out? They all believed it was impossible. I told them that I would give them the materials for an experiment and that they were to try to keep the penny in the cup when the cup is upside-down. I pointed out that attached to each cup is a piece of string – that’s a hint.

We then went outside and let them each play with a penny, cup and string. Some of them learned by swinging the cup that the penny would stay inside; some of them twirled the cup on the string; and, of course, once most of the kids had figured it out they all saw how easy it was. There was a great joy of discovery.

I proceeded to give them the next challenge. They all exchanged the penny for a ball. This one went fairly quickly. They all had fun exploring this new concept with a different prop.

When they had all mastered the second challenge, I explained that they now had to keep water in the cup. There were a few doubters, but they all were excited to give it a try. Everyone was successful and they kept at it till most of the cups had gotten soggy or fallen apart from the strain.