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Activity #2: Designing Counterweights

Visualizing and Describing The Science of Totally Trebuchet

Bigger, Stronger and Heavier

Ancient trebuchet engineers learned to build trebuchets the hard way, through trial and error.

As castle defenses were fortified, trebuchet builders responded by constructing increasingly larger

trebuchets. This meant more massive counterweights were needed to overcome the inertia of the bigger

beams, and hurl heavier payloads higher and farther.

The following problem requires a young trebuchet-engineering apprentice to calculate the interior

dimensions of a counterweight bucket. The young apprentice is required to furnish a drawing of the

bucket design showing the interior dimensions. Let’s see if we can use our understanding of mass, density

and weight, along with our technology skills, to help the young apprentice succeed.

The bucket must meet these specifications

Overall Weight: Maximum 25,000 lbs (11,339 kg)

The apprentice is to choose which of these two counterweight materials to use. The interior

bucket shape must be a cube, and it must be as small as possible and still meet the maximum

weight requirement.

Counterweight Material

Option # 1 Concrete with medium to heavy aggregate

Option # 2 Cut and stacked granite.

Note: Cut and stacked granite will have voids amounting to 10% of the total granite volume.

Here is one way to arrive at a solution:

1. Research ancient trebuchet and modern large-scale models to learn about counterweight bucket

form factors and construction methods. You can use books, videos or web sites. Since websites

change often, it is best to begin your search using your favorite search engines and keywords like:

Trebuchets, Trebuchet counterweights, Trebuchet construction, etc.

2. Decide on which system of units you will use. Choose either SI or Imperial and stick with it!

3. Research the density of medium to heavyweight concrete.

4. Research the density of granite.

5. Research the density of oak. Note: Published wood densities for specific species vary. Choose an

average density.

6. Build a spreadsheet that you can use to solve for volume, density and weight.

7. Find and use an online or published conversion chart to convert different density and weight

units.

8. Using the density data, and the required weight (25,000 lbs or 11,339kg), calculate the interior

bucket volume for each material.

9. Create a drawing of the bucket. Assume that it will be made from 6Ǝ thick oak planks.

10. Create your design and check your volume and weight using SolidWorks or other CAD software.

11. Provide three-view working drawings and solid model designs of the counterweight. Provide

mathematical proof of the volume and weight of the bucket. Create your finished design and

check your volume and weight using SolidWorks.

Sample Science Curriculum