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

