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Lesson Plans: Science > Phyiscs > Grass Ropes: Rope Structure and Tensile Strength

Playing tug-o-war with a grass rope.

Grass Ropes
By Thomas J. Elpel

Grade Levels: This lesson plan was designed for students in grades 5 through 12. The broad age range reflects the near-universal applicability of the concepts and activities: Making a grass rope is an excellent way to intuitively integrate academic lessons at any age.

Objectives:
      1) Help students understand rope structure by twisting short, weak fibers together to produce a long, strong rope.
      2) Provide students with an experiential understanding of tensile strength.
      3) Foster a sense of cooperation and teamwork as students construct and test a rope made of grass.
      4) Foster integrated, whole-person learning to connect the body and mind. Students develop skills and coordination to physically assimilate and compliment academic knowledge.

Background Information: Rope or cordage is a bundle of flexible fibers twisted together to increase its overall length and tensile strength. Cordage is a general term that applies to thread, string, or twine, while larger diameter products are generally known as rope.
      Ropes and cordage have been used since prehistoric times for daily activities such as fishing, trapping, hunting, carrying, lifting, and climbing. The tensile strength of a rope (or any object) is the amount of stress it can handle without breaking from being pulled apart. A rope in a game of tug-o-war is under tensile stress. So are the cables of a suspension bridge. In this lesson plan, students experiment with rope structure and tensile strength through the process of making a rope from grass.

Caution: 1) Avoid grass cuts by cutting grass with grass clippers, rather than pulling by hand. Grass cuts are not usually a problem while making or testing the rope. 2) Choose a safe location for playing games of tug-o-war, such as a lawn. 3) We have had students tow a school bus down the driveway with grass ropes. It is important to have a driver with a foot ready on the brake to avoid gaining too much momentum. 4) Do NOT attempt to play jump rope, as the mass of the rope can suddenly knock the feet right out from under a child.

Materials Needed:

  • A source for long grass, preferably green
  • Grass clippers to collect the grass
  • An outdoor space to make a test the rope
Also recommended (duplicates some of the information here):
Making the Rope: To make a grass rope, start by laying the grass out in two parallel lines, like train tracks. Each side should be uniform in thickness and about an inch thick when squeezed in the hand. For additional strength, go down each row and shuffle some of the grasses back and forth, so that the fibers are well mixed from end to end.
      Next, line up the whole rope-making crew shoulder to shoulder in two lines facing each other beside the grass. Put a capable or experienced person at the beginning of each line as designated line leaders. It is possible to make a rope with only three or so people on each side, but more is better. Ideally, there should be enough people to pick up both long lines of grass.
      Ask each line to step in towards each other, so that the lines are about two feet apart. Next, demonstrate the rolling technique. Start one line rolling the grass "away" in unison. Stop them when they have the proper technique. Then have the other line roll their grass "toward" themselves. Both lines of grass should be rolling the same direction, but the hand motion is different since the people are facing each other. (See the YouTube video for clarification.)
      Now, bring both lines together at the head of the rope. Pick up a handful of additional grass and kink it in the middle to make a "V," connecting both lines of grass. Start rolling both lines of grass. The two line leaders control the thickness of the strands as they come together. Stop the group as necessary to add or remove material to keep both sides uniform. The two line leaders move with the rope as the two strands are twisted together. Other people in the line will get squeezed out as the line leaders take over their positions.
      An additional person is needed to help hold and turn the new rope. It should slowly spin in the same direction as the two lines. Turning the rope will help keep it tight, so it won't unwind on one end while it is being wound up on the other.       If there were not enough hands to pick up all the grass at the start, then send anyone who gets squeezed out to the end of the line to pick up more grass to add to the rope. If all the grass is up in the air, then send the extra hands to the completed portion of the rope to help keep it spinning.
      When the grass is used up or the rope is long enough, then stop and tie an overhand knot at the end to keep it from unraveling.

Testing the Rope: The strength of the finished rope depends partly on the quality of the grass, and particularly on the uniformity of the rope. The rope is only as strong as its weakest part, which can be exceptionally weak if one strand is thick, with the thinner strand wrapping around it. But these ropes can be amazingly strong if uniformly made from end to end.
      The best way to test grass ropes is in games of tug-o-war. We have had some ropes that could not be broken by a class of 30 fifth graders. We had one rope, made of long, thin mountain grasses that could not be broken by a similar number of adults.
      Grass ropes can also be tied from a tree branch to make a good rope swing. The longevity of these green grass ropes is limited. They typically dry, shrink, and break within a couple of days. But kids can have a screaming lot of fun with them while they last.
      For longer-term ropes, look for non-brittle dry grasses or sedges that can be twisted without breaking. Grass ropes were commonly used for building suspension bridges among the Inca of Peru. One bridge, the Q'eswa Chaca (or Keshwa Chaca) is still rebuilt every year according to ancient customs.

Discussion: Tensile strength of the rope depends on a) fiber quality, b) fiber quantity (overall thickness), c) the number of strands being twisted together, d) the degree of twist, or lay, of the fibers, and e) consistency during manufacturing.
      a) Fiber quality: Natural fibers for manufacturing rope include hemp, sisal, cotton, flax, and jute. Modern ropes may also be made from synthetic materials, which have been spun into fibers or extruded into long filaments. Grass is a comparatively weak fiber, but makes a suitable rope for playing games.
      b) Fiber quantity: A thicker rope is generally stronger than a thinner rope made of the same materials.
      c) Number of strands: Fibers are twisted into at least two strands to make rope. More strands make a stronger rope.
      d) Degree of twist: A tight twist, or lay, makes a stronger rope than a loose twist.
      e) Consistency: A rope is only as strong as its weakest point. Consistency in manufacturing, such as maintaining an even diameter and uniform twisting, is essential to making strong rope.

Assessment: Students absorb practical, physical understanding of tensile strength and rope structure through hands-on experience. Conceptual understanding of can be assessed by standardized science book tests.

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