07 March 2011

spaghetti-mallow towers

This might be my favorite project for 7th graders. I use it during the construction unit in my CTE class. It's very cheap to do, it takes less than two days, the kids really enjoy it, and it teaches physics, math, engineering, manufacturing, and construction.

I usually introduce the project by talking about materials and forces. I take a piece of spaghetti and squeeze it from the ends to show the bowing. This is an example of compression. Too much compression and it snaps. Then I take a marshmallow and stretch it from the ends. This is tension. Too much tension and it simply pulls apart. Then I take a pre-made square of small spaghetti pieces attached at the corners with marshmallows, and I demonstrate sheer forces. Finally I take a triangle of spaghetti and marshmallows and demonstrate how they all interact. The force of gravity from the top will compress the side pieces of the triangle, which pulls apart at the base.

This little demonstration, I've found, is quite crucial to the students' production of a quality tower. It only takes a few minutes, but students see firsthand what kinds of forces they'll be facing and what they'll have to do to accommodate them. I also emphasize that the idea/design might be sound, but if it's built poorly, it will not be effective.

Anyway, here are the materials I use:

  • spaghetti (20 pieces per group)
  • small marshmallows (30 per group, there are about 500 in a bag)
  • large marshmallows (optional, 1 per group)
Be sure not to open the marshmallows until the day the students are actually building, and have the students do all the building in that first day. The marshmallows will dry out in a day, which makes them impossible to build with; however, if they dry out after being a part of a structure, they actually strengthen it.

Sometimes if a group completely destroys all their spaghetti and/or marshmallows, I give them the chance to start over for partial credit. Sometimes they end up building the best tower in the class with this second chance, sometimes they fail again...

Here's a video I made of some of the tower crushes. Students loved seeing it in slow motion. It also gives another chance to point out compression, tension, and sheer forces working on the towers.

video

I'm still working out how to work out the points. So far it goes like this:
  1. How tall is the tower? (in centimeters)
    • <3cm = 1 point
    • 3 - 3.9cm = 2 points
    • 4 - 7.9cm = 3 points
    • >8cm = 4 points
  2. What is the dead load of the tower (how much the tower itself weighs, in grams)?
    • >35 grams = 1 point
    • 26 - 34 grams = 2 points
    • 16 - 25 grams = 3 points
    • <15 grams = 4 points
  3. What was the maximum live load? (How much the tower held without breaking, in grams)
    • <400 grams = 1 point
    • 401 - 1000 grams = 2 points
    • 1001 - 1700 grams = 3 points
    • >1700 grams = 4 points
  4. Calculate the efficiency of the tower. Live load divided by dead load, multiplied by 100. (It's a percentage.)
    • <1999% = 1 point
    • 2000 - 2999% = 2 points
    • 3000 - 10,000% = 3 points
    • >10,000% = 4 points
Revision: 1 Feb 2012
Maybe I'm getting lazy, or maybe I'm just more efficient. But that old grading style was just too confusing for the kids to know what to go for. So I came up with a much simpler grading scheme:
  1. Students must use the supplies given to build a tower at least 5 centimeters tall, that can hold my textbook (which is just over a kilogram) in order to receive full credit.
  2. Students who fail to do the required height and strength will receive most of the credit for participation (unless they were rowdy or disrespectful somehow, then they'll get less).
  3. The tallest tower that still holds the textbook receives a prize or extra credit.
  4. The strongest tower that's still over 5 cm tall receives a prize or extra credit.
That's it. Since switching, it's just made everything a lot easier. This is an introductory class. They just need a taste of the engineering/construction.

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