The Planetarium's Earth-Center Model

A New Earth Exhibit

At the Northstar Planetarium, visitors can walk inside a round theater. The theater team is building an Earth exhibit that will help students investigate motion near Earth's surface. In a trial, a student holds an object above a soft catch mat. The holder keeps the object still until the student opens it. The student does not push the object. A camera records the object's first movement after it is released.

The team tests three objects: a wooden cube, a rubber ball, and a metal washer. They release each object from two platforms at different places in the theater. For every release, the platform height, holder, and catch mat stay the same. The team records the direction of the first movement, not the direction of a later bounce. A nearby fan is turned off so moving air is less likely to affect the records.

The team then makes a small globe model. A dot at the center represents the center of Earth. Dots on the globe represent places on Earth's surface. At each surface dot, the team draws a short arrow from the dot straight toward the center dot. The team calls the direction from a surface place toward the center local down. The model can be turned, so a place that is at the top of a page can move to a different part of the page.

The records below are made-up results from the exhibit's first set of trials. Each object was released three times at Platform A and three times at Platform B. “Toward the floor” describes the first movement seen by the camera.