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Secrets of Lost Empires I—Stonehenge

Classroom Activity

Objective
To investigate center of gravity.

• copy of "The Great Trilithion Balancing Act" student handout ( HTML)
• copy of "Great Trilithion Template" student handout (HTML)
• string, 15 cm (6 in.) long
• washer
• cardboard (about .1 mm, or 1/32 in., thick)
• scissors
• several large paper clips
• pencil with flat eraser
• hole puncher
• ruler

1. In this activity, students will locate the center of gravity in a cardboard trilithon.

2. Give each pair of students a copy of the "The Great Trilithion Balancing Act" and the "Great Trilithion Template" student handouts and a set of materials.

3. Explain that if an object is suspended at a single point, the center of gravity will hang directly below or at that point. Students can use a string attached to a washer to construct a vertical line beneath a point of suspension. They will then choose another point and construct a second line. The center of gravity is where the two lines intersect.

4. Next, have students remove part of the cardboard trilithon and locate the center of gravity again. The center of gravity is now located outside the object, between the two upright stones.

5. Finally, have students change the distribution of weight in the object by adding paper clips in such a way that the center of gravity is moved back to a location on the cardboard.

Because the cardboard Great Trilithon is an asymmetrical shape, students should discover that the center of gravity does not lie in the geometric center. When students remove part of the top of the Great Trilithon, they will change its weight distribution. They'll discover that the center of gravity has moved to a point outside the object, in the empty space between the two upright stones. Ask students to think of other objects for which the center of gravity is located where there is no material (such as a hollow basketball, an empty cup, or a boomerang). In order to move the center of gravity back to a location on the cardboard, students need to add paper clips as weights across the top of the horizontal stone (near the area where they removed part of the cardboard).

Point out to students that they used a cardboard trilithon to investigate center of gravity because of its asymmetrical shape. An actual stone trilithon is made of three individual stones and would not be balanced in this way.