Activities Index | Handout | Educator Ideas
(per group of four)
60 cm (about 2 ft.) each of several cables, such as yarn, thread, dental floss, and fishing line
2 empty 2-liter plastic bottles with caps, or a metal bucket with
2 pipe cleaners, or a metal "s"-hook
broomstick or pole
sand, salt, or water
For younger kids or kids with fine-motor difficulties, you may want to tie loops in the cables ahead of time to reduce the time needed for this activity.
After the activity, use "Golden Gate" from Bridges to show some ways to make cables stronger. (See the Program Description to locate this show segment .)
Try the Forces Lab
For more information, see Additional Resources.
Introduce the term "cable" by discussing elevators. Hold up a piece of yarn, string, or fishing line. Ask: Would you ride in an elevator hung from cables made of this material? (Kids will probably say no.) To show that it is surprisingly difficult to break the material in tension, wrap the ends of the cable around two pencils and pull the pencils apart. (The pencils keep you from hurting your hands.) Now introduce the activitymeasuring just how much weight different cables can support.
Lead the Activity
Instruct kids to pour the sand only as fast as it passes through the funnel. This prevents clogging the opening and allows more accurate measurement of how much sand the cable can hold before it breaks. If you are using water, tell kids to support the bottle as they add each cupful of water, and then replace the bottle cap before letting go of the bottle again. This will prevent spills when the cable breaks.
As the load gets heavier, have kids move the desks or chairs closer together to ensure that the broom handle doesn't break. If the soda bottle touches the floor before a cable breaks, the pole can be raised and held by hand.
The Big Idea
Unlike many other parts of structures, which experience combinations of compression and tension, cables support loads purely in tension. Thicker cables are not necessarily stronger than thinner cables. The particular material of the cable, as well as how the cable is formed, determine its strength and stretchiness. In this activity, kids will likely find that sewing thread is relatively weak and that fishing line is strong, with yarn and dental floss falling in between. Kids will probably be surprised at how much weight the different cables can support.
Different uses require cables with different degrees of stretchiness. For example, the non-stretchy wires that support tall radio towers keep the towers from moving too much. On the other hand, because ship mooring cables are stretchy, they can act as shock absorbers during storms. A fishing line's stretchiness allows it to absorb a sharp but short jerk that would snap a non-stretchy cable.
Build on It
Possible outcome: Kids may try twisting or braiding several lengths of cable or wrapping different kinds of cable together. Draw comparisons to the actual methods used in suspension bridges and elevator cables. (You may wish to show the video segment suggested above.)
Music In many musical instruments, putting metal or gut strings into different levels of tension creates a range of sounds. Kids can experiment with rubber bands stretched across pushpins on a board to find the right lengths to produce an eight-note scale.