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Activities Index | Handout | Educator Ideas
What's the strongest cable?
Cables are used to support hanging loads such as roadways and elevators. A hanging load pulls on the cable, putting it in tension. So, a good cable should be very strong in tension.
What You Will Need
• 60 centimeters (about 2 ft.) each
of several cables, such as yarn, thread, dental floss, and fishing line
• 2 empty 2-liter plastic bottles
• 2 pipe cleaners
• broomstick or pole
• dishpan
• measuring cup
• funnel
• sand, salt, or water
Make a Prediction
Predict which cable you think is strongest. Explain why you think so. How many cupfuls of sand in a soda bottle do you think this cable can support before it breaks?
Try It Out
1.Tie one end of each cable to a broomstick using the knot shown below.
2. Tie a loop at the other end of each cable. Hold the loop open with one finger while you tighten its knot.
3. Now make a load tester. Wind a pipe cleaner tightly around the neck of a plastic bottle. Form the other end of the pipe cleaner into a hook.
4. Place the broomstick between two desks or chairs
so the cables hang down. Hang the load tester from the bottom loop of the first cable, and wind the pipe cleaner tightly around itself. Place a pan underneath the bottle.
5. Place a funnel in the bottle. Slowly add the
load (sand, salt, or water), one cupful at a time.
Record how many cups you can add before the cable breaks. (You may need to add another
hanging bottle.)
6. Empty the bottle and repeat steps 4–5 with the next cable.
Explain It
• Which cable held the most weight? Compare the strongest and weakest cables. How are they different? Compare the two strongest cables. How are they alike?
• Which cables stretched before breaking and which did not? Can you think of any situations where a stretchy cable would be helpful or where it would be a problem?
Build on It
Can you design a way to make your strongest cable stronger? You can use any of the materials from this activity. How much weight can your improved cable hold before it breaks? How did your design make the cable stronger?
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