Round and Round

[30 Minutes]

As spring begins, we are all looking forward to breaking out the bicycles, roller blades and ball games. When you dust off the hula hoops and pop take out your croquet set, gather your family around and teach them a lesson. This activity will teach you about a scientific phenomenon called “inertia.” Your family will have fun, and they will learn something new from a great teacher…you!

You'll need:

• a collection of round, rolling objects: • spheres: solid balls of variety of sizes, such as croquet balls, marbles, golf balls, or marbles (remember, do not use hollow balls) • discs: large and small plastic checkers, drink coasters, Frisbees, heavy plastic dinnerware • hoops: hula hoops, spare tires, anything in the shape of a ring • a slanted driveway, hill, or a large slanted board

What to do:

1. If possible, print out these directions. Read them through with your child before you begin.
2. With your child, gather everything you need.
3. Let your child choose three people, if possible, to be in a contest.
4. Explain that you want to see which type of object will beat the others when rolled down a slope.
5. Ask your child if she would like to be in charge of spheres, discs, or hoops, and let her assign someone to the other two groups.
6. Ask your child questions, such as, “Which objects do you think will go fastest?” and “Why?”
7. At the same time, let three objects, one from each group, roll down the slope.
8. Continue choosing different objects, always choosing one from each group.
9. Who always wins?

Useful information:

You may have found that all spheres will always beat all discs. You will also find that all discs will beat all hoops, no matter what size you use. The speed at which an object rolls, is related to the distribution of weight around that object’s center of gravity, which is known as its “moment of inertia.” The solid spheres have the mass closely distributed around its center of gravity, so it has a small moment of inertia, and it rotates fast. The hoops have their mass distributed farther away form the center of gravity, so they have a large moment of inertia, and they rotate more slowly. Think of this principle the next time you watch an ice skater spinning in circles on the ice. She will start out with her arms extended, but as she pulls her arms in tight to her body, she spins more rapidly. As she decreased her moment of inertia, she increased her velocity.