```Balance and GravityStevenson, Mark                          James H. Bowen High School                                          933-7200                           Objectives:

Students will learn the basic concepts of balance and how to determine
the location of the center of gravity using a model of the leaning
tower of pisa and various other items.

Apparatus Needed:

plexiglass (5cm x 20cm)                triple-beam balance
string (35cm)                          beaker (250ml)

Recommended Strategy

Have students examine the triple-beam balance and weigh several
items. Ask them to notice what are the results of their measurements
when the balance is in equilibrium.  Raise the question; Where is the
center of gravity found using this apparatus?

Make a model of the leaning tower of Pisa from the plexiglass. The
model should be slightly larger at the base. Explain that the center of
gravity is an imaginary point at which we can consider the entire
weight of an object to be concentrated.  Place one hole in each of the
top corners of the model. Suspend the model from one hole at a time
while allowing the string and weight to come to rest below the base.
Draw a line from the point of suspension to the base.  Repeat this
procedure for the opposite side. The X where the lines intersect marks
the center of gravity. Place another hole at the point determined as
the center of gravity. If you now suspend your weight-on-a-string so
that it hangs freely from the center-of-gravity point and lean the
tower against a wall while its base stands on the edge of a book, you
can find out just how far over it can go without falling.  As long as
the line remains within the base line of the tower, it will not fall.
The instant the line passes outside the base, it falls. Explain to the
students that the reason the leaning tower of Pisa has not yet fallen
is because its center of gravity still lies within the base.
The lower the center of gravity, the farther over an object can
lean. Thus, by concentrating the weight of buildings, cars and boats as
low as possible, designers make them more stable.
Ask the students to bring to class at least one item which
demonstrates balance or the center of gravity for class discussion.
```