Stress and Strain

Therese Donatello              Weber High School
                               5252 W. Palmer St.
                               Chicago, Il. 60639
                               312-637-7500

Objectives:

Grade level- Junior High

1. To show the stress and strain involved in the movements of the earth's crust
   with the use of working models.

2. To show how the forces of compression, tension, and shearing effect the 
   movement of the earth's crust.

3. To show how materials effect the movement of the earth's crust.

Materials needed:

The following materials are needed to construct each model:

1. compression model - one piece of wood (30 cm x 15 cm), two pieces of wood (1 
   in. x 2 in. x 30 cm), two pieces of wood (1 in. x 2 in. x 15 cm), two bolts 30 
   cm long, and two wing nuts; 

2. tension model - one piece of wood (30 cm x 15 cm), two pieces of wood (1 in. 
   x 2 in. x 30 cm), two pieces of wood (1 in. x 2 in. x 15 cm) 2 pieces of screen 
   (15 cm x 10 cm), two screw hooks, and string; 

3. shearing model - one piece of wood (30 cm x 15 cm), two pieces of wood (1 in. 
   x 2 in. x 30 cm), two pieces of wood (1 in. x 2 in. x 15 cm), 2 grooved pieces 
   of wood (7 cm x 20 cm x 1 in.), two screw hooks, and string; 

4. each model also requires nails, screws, clay, styrofoam, rubber foam, 
   toothpicks, and masses. 

Strategy:

INSTRUCTOR'S PREPARATION:

1. The models were made by attaching the two 30 cm pieces and one of the 15 cm 
   pieces to the baseboard. 

2. On the compression model drill two holes the size of the bolts in each of 
   the 15 cm pieces.  Pass the bolts through the holes and attach with the wing 
   nuts. 

3. On the tension model attach the screen to the 15 cm pieces of wood.  Attach 
   the screw hooks into the movable 15 cm piece of wood and place the string 
   through the hooks. 

4. On the shearing model place the grooved pieces of wood on the baseboard and 
   attach screw hooks to the opposite ends of each grooved piece.  (It may be 
   necessary to place a thin strip of wood or plastic along the 30 cm sides to keep 
   the grooved pieces on the baseboard as they move.)  Place the string through 
   the holes of the screw hooks. 

5. The styrofoam, rubber foam, and clay are placed in or attached to each 
   model.  The toothpicks are placed in each of the materials. 

CLASS ACTIVITIES:

1. On the tension and shearing models the students are to place masses on the 
   strings in equal increments and note the movement of materials being tested.

2. On the compression models the students are to turn the wing nuts on each 
   side at an equal rate and note the movement of the materials being tested.

3. The force can be calculated mathematically with advanced classes or spring 
   scales that read in newtons can be used in place of the masses.
  
Expected results:

1. On all the models the students should note that the clay requires more force 
   to move or break than the styrofoam or the rubber foam.

2. On all the models the toothpicks in the clay not only move apart or together 
   but also move at angles to one another showing the forces at work below the 
   surface.  (It was suggested that other models could be made with plastic sides 
   to show the movement of the layers below the surface.  Also by using different 
   colors of clay or foam the sub-surface movement might be more visible.)  
 
Conclusions:

     The movement of the earth's crust is caused by convection currents below the 
surface.  This movement causes some plates to come together as in the case of 
India and Asia forming the Himalayas, while other plates move apart as in the case 
of the Mid-Atlantic Ridge, while still others move along each other as in the case 
of the San Andreas Fault.