physics balsa wood bridge project

Physics – Toothpick Bridge Project
Construction of your bridge begins during class. It is your group’s responsibility to
bring a large box to class with a lid to store your bridge for safe keeping (two shoeboxes taped together is
perfect), flat toothpicks, white glue, and wax paper. No more than two people will be allowed in one group.
You will probably need to take the bridge home a few times to get it done. You may NOT do the bridge
completely at home – you must work on it during classtime and only take it home as needed to finish it.
10-14-2012-M through 10-16-2012-W
We will test bridges during class. Your group should sign up
for a date to test on, and your bridge should be completely done 24 hours before this date (for optimal glue
drying time). Sign-up sheets will be available starting October 9 on a first come first choice basis.
Your bridge should hold at least 5 pounds to score a “B” on the project.
Your bridge must span a gap of 40 cm. This means that if you make your bridge 40 cm long, it will
fall through. See diagram.
The bridge can only rest on the top surface of the supports and against the inside of the vertical
supports for no more than 8 cm from the top.
The bridge must be at least 5 cm wide throughout its entire length and no more than 15 cm wide.
The bridge must provide a place for the 5 cm testing belt to go around it. All weight will be placed at
this point. See diagram.
You may only use flat toothpicks and white glue.
Bridges may not be painted, varnished, or over-glued. See diagram for over-gluing.
Toothpicks may be shortened, blunted, or bent.
The finished bridge must weigh less than 450 g (one pound).
Bridge Scoring
Your bridge’s score will be based on more than just how much it can hold. It is easy to build a heavily glued
toothpick bridge that holds a lot of weight. It is much more challenging to build a lightweight bridge that
holds more than its own weight proportionally. Your score will be a ratio of how much your bridge holds
compared to how much your bridge weighs. The formula is as follows:
(Bridge’s failure load in kg)  (Bridge’s mass in g) x 1000 = your score
You are working in class on your bridge – participation
0 -------------15
Type 3 follow up writing (details will be coming)
0 -------------10
Your bridge’s final score
Points awarded
Holds the minimum only
7 – 80
81 – 175
Extra credit
Total points possible
Find a way to distribute the weight to the supports.
Hairdryers and fans make glue dry faster.
Nail clippers can be very useful.
A pair of tweezers can also be very useful.
The more “directions” your toothpicks are going in, usually the better. (i.e. IIIIIIIIIIIII is much less
strong than XIXIXIXIX and 3-D is even better.)
Soaking toothpicks in water will let you bend and weave them.
A shorter toothpick is stronger than a longer toothpick.
Glue toothpicks together to form beams.
Always worry about the stress points, especially around the supports.
Your bridge should hold at least 5 pounds to score a “B” on the project (assuming you earn full
participation points and writing points).
Acceptable and unacceptable gluing
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