Key concepts
Physics
Geometry
Engineering
Trusses

Introduction
Have you ever driven across a bridge or seen a building that is under construction and noticed the large metal support girders? What about wooden beams in a house that is under construction? Did you notice how sometimes the supports form different geometric shapes such as triangles or squares? In this project you will be a structural engineer and make your own "support" shapes out of popsicle sticks. What shape do you think will be the strongest?

Background
When civil engineers build large structures such as bridges and buildings, they have to take into account how forces—such as gravity, compression, tension and torque—will act on the building materials. Gravity is constantly working to pull materials—especially heavy ones—toward Earth. The weight of cars driving across a bridge, for example, can push (creating compression) or pull (tension) on a structure, causing them to break if they are not designed properly. Engineers also have to design structures to handle torque, or "twisting." An example of torque is when you turn a screwdriver to twist a screw or if you were to hold a ruler in both hands and try to bend it.

Bridges and buildings usually have their frames built as a "truss," or a series of beams that are connected at their ends. The engineer's goal is to design a truss that will slightly flex but not bend or break—even with strong forces acting on it. In this project you will make trusses by connecting popsicle sticks end to end with binder clips. Can you guess what simple geometric shape will resist bending the most? Get ready to find out!

Materials

  • Popsicle sticks (at least seven)
  • Small binder clips (at least seven)

Procedure

  • Clip two popsicle sticks together end to end using a binder clip.
  • Hold one end of each popsicle stick in each hand. Gently try to twist them back and forth, rotating about the joint where they are connected by the binder clip. (Do this by sliding the flat surfaces of the popsicle sticks against one another, do not try to "break" them by bending them.) How easy is it to rotate the popsicle sticks about the joint?
  • Now make a square out of popsicle sticks that are connected by binder clips at the four corners.
  • Grip two adjacent popsicle sticks with your fingers and gently try to rotate them relative to the joint that connects them. How easy is it to rotate the popsicle sticks? What happens to the shape of the entire square?
  • Grip two popsicle sticks that are opposite one another and gently try to slide them back and forth parallel to one another. How easy is it to slide the popsicle sticks back and forth? What happens to the shape of the entire square?
  • Now make a triangle out of popsicle sticks that are connected by binder clips at the three corners.
  • Grip two adjacent popsicle sticks with your fingers and try to rotate them just like you did with the square. What happens? Can you rotate the popsicle sticks or does the triangle maintain its shape? How does it compare with the square?
  • Extra: If you have more popsicle sticks and binder clips, try making a larger truss structure out of multiple connected squares and/or triangles. What happens when you push, pull or twist different parts of the truss? Does one shape (triangle or square) tend to "hold its shape" better whereas the other one slides around?
  • Extra: Try adding a diagonal across the square, dividing it into two triangles. (Secure two popsicle sticks together with multiple binder clips to make a single, longer stick that cannot rotate). Can you still easily rotate the square?
  • Extra: If you have building toys available, such as K'nex, Tinkertoys or LEGO Technic pieces, try using them to make trusses of different geometric shapes. (Note that this will not work with regular LEGO bricks, you need beams with holes that can be connected by pegs.) Do you get the same results as you did with popsicle sticks?

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Observations and results
You should have found that it was very easy to rotate the popsicle sticks in your square truss. When you rotate two adjacent popsicle sticks, or slide two sticks that are opposite one another, they all stay connected but the entire square deforms to form a parallelogram—which might be fine for a popsicle stick structure but disastrous for a building!

When you try to rotate the popsicle sticks in the triangle-shaped truss, however, they do not move. The triangle design is "stronger"—the popsicle sticks are arranged in such a way in this shape that when you push or pull on them, none of them can rotate.

This happens because of something engineers call "degrees of freedom." The square truss has one degree of freedom, which means it can move in one direction (in this case, a rotational direction—it can rotate from a square to a parallelogram). The only way to prevent the square from rotating at all is to pinch down very, very hard at the joints (imagine using much stronger binder clips). You can now see why it would not be good to only use squares when building structures—all of the joints would have to be superstrong! The triangle, however, has zero degrees of freedom—all of its popsicle sticks are fixed in place and cannot rotate. This means you can build a truss structure out of triangles that does not rotate or deform—even though the joints are only lightly held together by binder clips.

More to explore
The Effect of Bridge Design on Weight-Bearing Capacity, from Science Buddies
All about Bridges: Importance and Types, from Easy Science for Kids
Sphere-Based Science: Build Your Own Geodesic Dome, from Scientific American
Science Activities for All Ages!, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies