You and Your Students!
Vicki Cobb, Education World Science Editor
Take string and rope apart to discover their strength.
Setting the Scene (Background)
When I was knitting the other day I was reminded about what an amazing invention string is. The individual fibers are very tiny and very weak. But when they strands of fine fiber are combined, they work together to create a strong bond. The secret is in the twist
One of the best ways to understand something is to take it apart. Have your students bring in pieces of string, twine, yarn, thread, rope -- a variety of cordage. This is the kind of hands-on activity in which everyone can get involved. Give each student at least two different kinds of cordage cut into 2-inch lengths. Have them work at their desks as you describe what they are to do.
Have students examine their cordage samples. What do they have in common? With rare exceptions, all cordage is twisted That twist can be in one of two directions. To figure out the direction of the twist, grasp opposite ends of a cord. Hold one end firmly with the less favorite (weaker) hand. Use the favored hand to rotate the other end of the cord between the thumb and first finger.
Note: This is a good opportunity to review with students the directions "clockwise" and "counterclockwise."
Different kinds of cordage have different amounts of twist. You can see this difference by comparing the number of times you turn over one end of a piece of cordage to unwind it. Many cords will separate into strands, or plies, as you unwind them. The most common cordage is three-ply (with three strands). Often the strands themselves have been twisted. Check the direction of the twist in the strands. If the cord has an S-twist, the ply often has a Z-twist.
If you continue to untwist the strands, eventually you'll end up with a single fiber. Examine that fiber with a magnifying glass. Some fibers will be very smooth and fine. Some will be coarse, with an irregular shape. Have the students pull on the fibers to break them. Some will be stronger than others.
Some cordage can be cut without unwinding. If you simple twist two threads together they instantly unwind when you let go. So how do manufacturers make the twist permanent? If you've ever done a lot of hand sewing, you've seen the answer. Your students can see it, too. Have them work in pairs.
You might notice this when you sew: Your over and over motion of the needle twists the thread so that it wraps around itself. Every once in a while you have to let the needle hang down so that the thread can untwist. Very annoying but there's no getting away from it!
Behind the Scenes
The art of rope-making depends on transferring the twist from one place to another. In colonial days people worked together "laying" ropes by hand. Each ply was given a separate twist, then three or four plies were stretched out side by side and released. As the plies untwisted they wound around each other. Today, ropes are made by machine; they are much tighter than any made by hand.
Article By Vicki Cobb
Copyright © 2005 Education World