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  Activity: Tug-Push-Twist-O'War
Activities Index | Handout | Educator Ideas  

20–30 minutes

(per group of three)
Samples of 6 or 8 different materials, zsuch as string or yarn; popsicle sticks; pipe cleaners; clay; strips of kitchen sponge; rubber erasers; rubber bands; paper-towel tubes; pencils; strips of cardboard or aluminum foil; drinking straws; ceramic tiles; strips of cloth.

Video Connection
After the activity, use "Iron Bridges" from Bridges, "U.S. Capitol" from Domes or "Eiffel Tower" from Skyscrapers to show how the choice of materials affects structures. (See Program Descriptions to locate the show segments listed above.)

Try the Materials Lab.

For more information, see Additional Resources.

Use a rope to demonstrate the three tests. Have two kids tug on the ends of a rope (tension), then push the ends together (compression), and finally twist the ends of the rope (torsion). With each test, have the group suggest a rating using the scale on the Activity Handout. (The rope is strong in tension but weak in compression and torsion.)

Lead the Activity
• Discuss how kids can fairly compare the different materials. Encourage them to Ūnd a consistent way of handling the materials. For example, besides pushing a material together between their hands, another way to test for compression is to place the sample on a tabletop and press down on it. 
• Discuss how some materials are flexible under a type of stress–they change shape as opposed to breaking outright. When might flexibility be desirable? When is stiffness required? (Parts of structures such as the cables of suspension bridges that are built to withstand shaking caused by wind gusts often have some "give." Other parts of structures, such as floor beams that support great weights, need to be rigid.)

The Big Idea
Different materials have varying abilities to withstand compression, tension, and torsion. Materials scientists study these properties of construction materials using machines that apply enormous loads to the samples and measure their ability to withstand the stresses that result. Results may differ somewhat due to the limitations of these testing methods, but in general kids will Ūnd results similar to these. 
Strong in tension: string, yarn, pipe cleaner, popsicle stick, ceramic tile, cardboard, drinking straw, cloth, rubber band (strong but very flexible), rubber eraser, paper-towel tubes, pencil 
Strong in compression: popsicle stick, clay (limited), ceramic tile, rubber eraser, paper-towel tubes (limited), pencil 
Strong in torsion: ceramic tile, rubber eraser (limited), paper-towel tubes, pencil

Build on It
Possible outcome: Kids can compare shapes and sizes of the same material, such as a flat piece of cardboard and a cardboard paper-towel tube. They will Ūnd that the curved shape of the cardboard tube increases its stiffness and resistance to compression and torsion.  
Try the Shapes Lab.

Make Connections
Language Arts: Have kids choose a material and write a paragraph from that material's point of view, explaining what it feels like to be in compression, tension, or torsion. Encourage kids to be creative and to use descriptive verbs and adjectives. Kids can team up to read aloud and act out each other's paragraphs.