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How many pennies can my file-card bridge hold?
You may find that a file-card bridge can hold more
pennies than you'd think! Here are the results of the file-card bridges
that the Science-at-Home Team built.
A roll of 50 pennies weighs 132 grams-that's a little more
than 41/2 ounces.
How many kinds of bridges are there?
You might think that bridges come in an infinite
variety of forms. But if you get right down to the structural elements of
a bridge, there are really only three kinds: beam spans, arch spans, and
suspension spans.
The simplest kind of bridge is a beam bridge. A log that
has fallen across a river makes a beam bridge. So does a board laid across
a puddle, or a span of steel laid across a body of water, or a file card
laid across two books. A beam bridge relies on the stiffness of the building
material. If the log across the river sags, it doesn't make a very good
bridge.
Arches have been common features in buildings since 1,000
B.C., but they didn't appear in bridges for another thousand years. Roman
roads, built at the height of the Roman Empire's power, were often supported
by stone arches.
Suspension bridges, like the Golden Gate Bridge in San Francisco,
rely on a cable or rope for their support. Each end of the cable or rope
must be anchored to the bank-tied to a tree, a boulder, or (in modern suspension
bridges) a massive block of concrete called an anchorage. The cable or rope
pulls on the anchors, but as long as they don't move and the cable or rope
doesn't snap, the bridge is stable.
What kinds of bridges can I make with my file cards?
Using just your file card, you can make two of
the three different kinds of bridges. When you lay a file card across two
books-even if you've folded the card into pleats first-you've made a simple
beam bridge. If you cut slots into the card, tuck the flaps under the edges
of the book covers, and push the books slightly together, you'll make an
arch bridge. We haven't figured out how to make a suspension bridge out
of a file card, though. If you come up with a way to do it, please let us
know!
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Make 2 stacks of books with a
gap of about 4 inches between them. Make sure the stacks are the same height.
Lay one file card over
the gap between the books. About 1/2 inch of the card should be resting
on a book at each end. How many pennies do you think you can pile on this
flat bridge before it falls into the gap-5? 10? 100? Try it and see how
close your guess was.
Without adding anything
to the file card, try to make your bridge stronger. How could you change
a file card to make it stiffer? What happens if you fold the card in half?
If you make an arch?
How about if you fold the card into pleats?
Make a bridge, then test
it to see how many pennies it will hold. Some of your bridges may hold a
few pennies before falling down. Others may be stronger, but the pennies
may slide right off. And some bridges will probably hold a lot more pennies
than you'd think.
