Balancing Stick

Place a lump of clay about the size of your fist around the dowel, about 8 inches (20 centimeters) from the end of the dowel.

Balance the stick on the tip of your finger, putting your finger under the end that’s near the clay. Now turn the stick over and balance it with the clay on the top. Notice that the stick is easier to balance when the clay is near the top.

The dowel rotates more slowly when the mass is at the top, allowing you more time to adjust and maintain balance. When the mass is at the bottom, the stick has less rotational inertia and tips more quickly. The farther away the mass is located from the axis of rotation (such as in your hand), the greater the rotational inertia and the more slowly the stick turns. An object with a large mass is said to have a great deal of inertia. Just as it is hard to change the motion of an object that has a large inertia, it is hard to change the rotational motion of an object with a large rotational inertia.

You can feel the change in inertia when you do the following experiment. Grab the end of the dowel that’s near the clay. Hold the dowel vertically, and rapidly move the dowel back and forth with the same motion you would use to cast a fishing line. Next, turn the dowel upside down, and hold it at the end that is farthest from the clay. Repeat the casting motion. Notice that it is much harder to move the dowel rapidly when the clay is near the top. The mass of the stick has not changed, but the distribution of the mass of the stick with respect to your hand has changed. The rotational inertia depends on the distribution of the mass of the stick.

Instead of demonstrating this Snack in advance, give a group of people the clay and dowel separately, tell them to attach the ball of clay somewhere on the dowel so the dowel sticks out at both ends, and challenge them to see who can balance the dowel the longest. Let them discover the role of the clay.