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Stretch-O-Meter Journal
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Science Lab
Stretch-O-Meter

Mimi Koehl is a biomechanist, a scientist who studies how living things work physically. She builds models and sometimes her own equipment to study how different organisms function in fluids like water or air. Mimi is an innovator because she approaches natural materials as an engineer. One of the things that an engineer does is test the stiffness of materials to see how they will hold up in an airplane, a car, or a bridge. Engineers ask questions like, "How stiff are the materials? At what point will they not rebound? At what point will they fracture?"

In this activity you will make your own Stretch-O-Meter and measure the stretchability of different materials.

What You Need
 
  • Empty, clean milk carton
  • Scissors
  • String or twine
  • Medium-size binder clip
  • Hole punch
  • 10 marbles
  • Ruler or measuring tape
  • Marker
  • Materials such as rubber bands, plastic wrap, a shoelace, yarn, cardboard, a twig
  • A fellow scientist
  • Stretch-O-Meter Data Sheet
  • Stretch-O-Meter Journal
    
Stiff Competition
What You Do image 1. Invite a friend.

Call up a good friend and invite her over. Then get to work!

2. Print this activity.

Print this page, the Stretch-O-Meter Data Sheet, and the Stretch-O-Meter Journal.

3. Make your Stretch-O-Meter.

Cut off the bottom half of your milk carton (you can throw the top away). Punch a hole in the middle of each side about an inch from the top. Cut four 46-centimeter (18-inch) pieces of string. Tie one end of a string through a hole in the carton. Then do the same for the other three holes. Fold the handles of the binder clip back. Gather two of the strings and tie them to one loop of the binder clip. Tie the other two strings to the other loop. Put 10 marbles into the carton. Your Stretch-O-Meter is complete!

4. Gather materials.

Find three objects you think will be stretchy and three you think will not stretch. For example, try different types of rubber bands, plastic wrap, yarn, a twig, cardboard, or a shoelace. You must have a piece of each material that is at least 10 centimeters (4 inches) long and at least 3 millimeters (one-eighth of an inch) wide.

5. Prepare your materials.

Cut a piece of each material 10 centimeters (4 inches) long. If the piece is more than 5 millimeters (one-fourth of an inch) wide, cut it to a width of 5 millimeters. Make two marks, 6 centimeters apart and 2 centimeters from the top and bottom of each sample of material (2 and a half inches apart and three-fourths of an inch from the top and bottom).

6. Measure the material's stiffness.

Clip the binder clip below the bottom mark on your material. Pick it up, holding it above the top mark. Raise it so the Stretch-O-Meter is not resting on a surface. Have your scientist friend measure the distance between the two lines marked on your sample. Write the measurement down on your data sheet. If the material breaks, write down "fracture." Repeat step 6 for each material you want to test.

7. Calculate the stretch.

Subtract the original length of 6 centimeters from the stretched length for each material. Write your answers down on the Stretch-O-Meter Data Sheet.

8. Remeasure the material.

Unclip the material from your Stretch-O-Meter. Then measure the distance between the two lines again. Write down your answers in the third column of the data sheet. Repeat these steps for every piece of material you work with.

9. Analyze it.

In your Stretch-O-Meter journal, write down some of the things you learned as you used your Stretch-O-Meter.

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