My boys love catapults.  Whether we are learning about early methods of warfare, watching Pumpkin Chunking’, or flinging marshmallows at each other – catapults are a constant source of engineering wonder.  Over the years the boys have built multiple catapults from kits as well as from scratch.

This year they were able to compete together in a Science Olympiad event called Ready, Aim, Fire!

The competition requires building a catapult of dimensions less than 65 cm x 65 cm x 65 cm powered only by a falling counterweight.  At the competition teams are given counterweights of 1 kg and 2 kg that are used to launch projectiles of 20-40g and 40-60g respectively.  Before the competition teams are expected to launch projectiles of various masses and record data.  The team prepares plots that are used to show where various projectile masses are expected to land.  On the day of competition the catapults are impounded.  After all devices are impounded, the judges announce the masses of the two projectiles that will be used that day.  Teams use their prepared graphs to determine how far they think their catapult will throw the projectiles and then tell the judges where they would like their target set in 0.5 m increments.

Launch scores (LS) are determined using the formula:

LS = TD – 3A +B

where TD is the target distance requested by the kids, A is the distance from where the projectile makes first impact to the center of the target and B is a bonus for hitting the target.  B = 0.15 x TD if the projectile hits the target and 0.30 x TD if the projectile stays in the target.   All distances are in meters.

Points are also awarded for graphing. (Up to 12 points).

This was a great project for the boys.  They handled much of the build themselves.  I helped out with a few of the cuts and my husband taught the boys to use the drill press.  My husband handled the original “basket” build with the boys making modifications later.

E recently had lessons in geometry and basic trigonometry, so he was able to use his knowledge of sin and cos to make the throw arm its maximum length while maintaining the optimum ratio of counterweight arm length to projectile arm length.   We also taught C the methods he was using.  It was a rather complex calculation so we set up a spreadsheet to do some iterations (side lesson on radians vs. angles was necessary).

The boys previous build experience came in very handy.  From the beginning they understood a trebuchet design would be best design for distance.  They also understood the importance of release angles and were quite keen at recognizing when the release angle was off and doing things to improve it.

Testing the trebuchet took quite a bit of time as they chose to make improvements to the basket and sling which required re-collecting all the data points they already had.  They used a spreadsheet to organize their data and select the trend line.

All their hard work paid off when they were able to make accurate predictions the day of the competition.  They were able to hit the target on the 1 kg launch and come very close on their 2 kg launch (the ball landed just a bit to the right).

It was a very exciting day for them.  They were thrilled to take home first place medals and they are already looking forward to competing in the middle school division next year.

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