BACKGROUND:
Geometric shapes help students to describe natural
objects. However, real things rarely have shapes as perfect as those created
by a ruler or a computer. This sometimes confuses children. They imagine the
perfect geometric shape, but the real object is imperfect. Learning
different shapes by using models thus enriches a child’s understanding of
shapes, but does not fully define the appearance of natural objects.
Students must learn that shapes are only guides to the characteristics of
real things.
Children naturally see the world in three
dimensions. At an early age, however, they should begin to distinguish three
dimensional shapes from their two dimensional equivalents. They must learn
that when three dimensional shapes are drawn on a flat surface, they usually
appear or "become" two dimensional. Since this adjustment is
natural for adults, they usually guide students’ understanding of this
distinction poorly. It is important to give students examples of equivalent
three dimensional and two dimensional objects. For instance, if you show the
students a globe, also show them a circle drawn on the board or on paper.
In this exercise, the students are introduced to
different shapes - a cube, a hexagon, and a circle. They will try and
determine which of these is most similar to the Earth’s shape. Most
children know that the Earth is spherical, but when asked, they will say the
two dimensional term, circle. Make sure the students begin to understand the
distinction between a sphere and a circle.
PROCEDURE:
- If you have a set of geometric models, you may wish to use them to
demonstrate different shapes to the class. Make sure to show the
students the two dimensional equivalents of the shapes as well.
- Have the students complete the upper part of the worksheet.
- Ask students how we know that the Earth is spherical. Most children
have seen pictures of Earth from space, or have seen a globe, so they
know that the shape is spherical. You can tell the students that
hundreds of years ago people thought the Earth was flat and that ships
could sail off the face of the Earth. People slowly began to realize
that ships coming from long distances appears from the top of the mast
before the entire ship could be seen.
- The lower part of the worksheet has students think about what is
inside the Earth. Evidence about this is more difficult to observe than
the shape of the Earth. Most students eventually arrive at the correct
answer, "rocks" by a process of elimination. This is basically
correct; the Earth is basically a ball of rock that is rotating and
revolving in space. The interior of the Earth is very hot, but there is
no fire per se within it.
- As an added exercise, you may want to give students playdough , and
have them fashion a spherical "Earth." Here are a pair
of playdough recipes:
PLAYDOUGH RECIPE #1 (the dough formed is not as durable as recipe
#2)
1 cup flour
1\2 cup salt
2 teaspoon cream of tartar
1 cup water
RECIPE #2. CLASSROOM QUANTITIES (alum helps preserve the
playdough)
5 cups of flour
1 cup of salt
2 tablespoons of alum
2 tablespoons oil
3 cups of very hot water
For both recipes, first mix together all the ingredients. Cook them over
medium heat while stirring, for 3 to 4 minutes, until a dough ball forms
and separates from the sides of the pan. You may add food coloring or
glitter for special effects. Adding food coloring works best when it is
added with the water at the start of the recipe.
