Build a Paper Bridge
Do you think you can you build a bridge out of paper? Try it out and then see if you can make it stronger with these tips.
What You Need:
- a sheet of paper
- a few thick books
- pennies or other weights
What You Do:
- Make two supports for your bridge by stacking books into two stacks that are the same height. They should rise several inches off of the table or floor and be about 6" apart from each other.
- Lay a plain sheet of paper across the stacks of books. There should be an even amount of paper over each stack.
- Place a penny, paperclip, pencil, or other light object in the center of the paper. Does your bridge support the weight? If so, add more until it collapses. If not, try to make it stronger.
- A sheet of paper is very thin and without anything to support it in the middle, it will collapse easily. Can you think of any ways to make your bridge stronger? You could move the supports (stacks of books) closer together, or try folding the paper in different ways to make it stronger. Here are some tips:
- Fold the sheet in half the short way so that you have a long, narrow piece. Tuck the ends under the flaps of the top book in each of your stacks and push up on the paper to make an arch (you may need to push the book stacks closer together). The extra layer of paper makes it slightly thicker and the arch shape can support more weight than a flat sheet of paper.
- Unfold the paper. Fold the short ends over twice to meet the center, so that you have 5 fold lines. Bend the paper up along the folds to make a four-sided box. Set it across the stacks of books and test it.
- Fold it accordion-style into a series of zigzags (you can start with a new unfolded sheet of paper). Start at bottom (short end) and fold the edge up a little more than an inch, then flip the paper over so that the folded flap is facing the table. Fold the already folded flap up the same amount. Flip the paper again and continue folding back and forth so that the folds make a zigzag pattern. Stretch the paper out slightly and set it across the space between the books. This triangular folding makes the bridge stronger, just like a truss bridge, because the weight is spread out more over the triangular shapes. This is why corrugated cardboard is stronger than ordinary cardboard - the folds inside of it make it more rigid. Try adding even more folds - does it make the bridge even stronger?
- Can you think of any other ways to fold your paper? Which makes the strongest bridge?
- Can you think of materials you have around your house that you could use to make a stronger bridge?
Arch Bridge Activity
Arches are one of the oldest types of bridge. Try this activity to learn more about them.
- Take a thin piece of cardboard that is longer than it is wide and bend it into an arch shape.
- Set it on the table, making sure it still has at least a slight arch.
- Press on the top of the arch. What happens?
- Now arch the cardboard again and place it between two stacks of books.
- Press on the top of the arch again and notice what happens this time.
The stacks of books act as abutments or supports to keep the ends of the arch from collapsing from the weight you put on it. Without them, there was nothing to support the weight of your hand and keep the ends of the arch from pushing outward and falling down.
For another simple project, use sugar cubes to build an arch. Taper the sides of the cubes with an old nail file (or scrape them with a butter knife) so that they fit snugly against each other to form an arch. Be sure to use an odd number of cubes, so that there's only one cube in the middle. If you used two cubes in the middle, the frictional force between those blocks would be overcome by the load force on the arch, causing it to buckle.
Suspension Bridge Activity
Make a model of a suspension bridge and learn why anchors for the cables are so important!
- Pick two hardcover books that are the same height and about the same thickness. Stand them up on end, facing each other, several inches apart.
- Tie a piece of string around the top of one of the books.
- Letting the string hang loosely between the books, tie the other end to the top of the other book.
- Now press on the center of the string suspended between the two books. What happens? (The books will fall over.)
- Untie the string and set the books back upright, about 10" apart.
- Place a stack of books on top of one end of the string. Pull the string tight and hang it over the tops of the standing books, with some string hanging loosely between them.
- Put the other end of the string securely under another stack of books, a few inches away from the standing book.
- Now try pressing again on the string in the center. What happens this time?
When the string was tied around the tops of the books, there was nothing to support the books when weight was applied to the loose string. When you changed the design by putting the ends of the string securely under stacks of books, the books helped anchor the string and stabilize the upright books. This model is very similar to how a real suspension bridge is designed. The books standing upright are like the towers at each end of the bridge and the string is like the cable that is anchored at each end. Depending on the size and thickness of the books you used, the books may still fall over, but they should stay up better than the first time you tried it (without the anchors). You can experiment more with different sizes of books, different kinds of strings, and even using two strings a few inches apart from each other. Take a look at the picture to the right - can you tell where the cables are anchored at each end of the bridge?
To learn more about bridges, visit this Teaching Tip.