To introduce the unit on building, my boys learned many factors that affect buildings in a hands-on way.They experienced compression by teams pushing their hands against each other's; tension by pulling against each other's curved fingers and torsion by hold each other's wrists with hands and twisting in opposite directions. We discussed how some materials are flexible under stress–they change shape as opposed to breaking outright. 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.
We demonstrated how to test materials themselves by doing three tests on a piece of rope. Two of the boys tugged on the ends of the rope to test its tension, then pushed the ends together to test compression, and finally twisted the ends of the rope to test torsion. With each test, we decided as a group what rating the material should get in each of the categories.
Then we started testing household materials that the boys had found the day before by way of a scavenger hunt. for these three qualities.
Sometimes it was hard to decide how to rank the different materials in terms of tension, compression and torsion, but we did make a chart of the results of our testing, ranking the items from 1 (weak) to 4 (strong) in the different areas. We will use this information when we later build various structures.
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
We also compared shapes and sizes of the same material. A flat piece of cardboard and a cardboard paper-towel tube were rated differently as the curved shape of the cardboard tube increased its stiffness and resistance to compression and torsion.
|Activities and photos from 6/18/08|