Learning with Catapults

Catapults from kitsMy 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!

IMG_0069The 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).

Boys with TrebuchetIt 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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What Went By : Elementary Science Olympiad

Today I’m focusing on resources to help you prepare for the “What Went By” event at an Elementary Science Olympiad.  If you have a child interested in animals and nature you will enjoy this list as well.
Wild Tracks! by Jim Arnosky is a fantastic resource.  It may look like a child’s picture book from the outside, but it is full of helpful information and LIFE-SIZE Prints.  Kids will learn that hoofed animals walk and run on their toes and wildcats use the same tracks over and over again, however this book stays away from scientific terms such as digitigrade and direct register.  Instead it is written in a friendly, digestible form perfect for independent learning even at younger ages.  The tracks examined are mostly common mammals of North America.

Animal Tracks and Signs by Jinny Johnson from National Geographic is another great resource.  The foreward does a nice job of explaining reasons a person may want to identify animal signs and tracks. The introduction provides a great list of scientific vocabulary terms that are useful for a young biologist and covered by the “What Went By” event.  The pictures of scat are very useful as most books don’t include color pictures of scat.  The scope of this book is very broad and contains information about animals from all over the world.

The Nature Series: Science on Tracking Expedition kit is also useful for kids interested in learning more about animal tracking.  The focus of this kit is purely tracking.  It comes with plaster of paris for making your own cast in nature or from the supplied molds.The matching cards that come with this kit are useful for memorizing different track attributes.  If you are creative you can create

National Geographic has a teacher’s guide that is pretty helpful when it comes to identifying scat.  We thought about doing the candy scat project, but decided we didn’t want to solidify those associations in our brains.

Our local natural history museum had a teacher’s resource kit that was very helpful.  You may want to check with your state conservation department or local park system to see if they have any kits available for loan.

In addition to the resources listed above, we found local nature centers did classes specifically about animal tracking with Science Olympiad preparation in mind.   These classes yielded some helpful hints we didn’t find in the books.


Perhaps the best preparation was just spending lots of time outdoors with an eye toward animal signs.

Mousetrap Car : Elementary Science Olympiad

This year our homeschool group put together a Science Olympiad team for the first time.  We were competing in the 4th to 6th grade division.

Our task was to design a car power completely by a mousetrap that would go EXACTLY 10 meters.  Every mm short OR long of 10 m would result in points.  Teams also gave a time estimate for completing a run.  Points were given for the difference between predicted time and actual time.  Keeping the centerline of the track between the wheels was worth a -20 point bonus.  The goal was to have the LEAST number of points.

IMG_1658The boys came up with an original design that works quite well.  The chaise is made entirely of LEGO Technic pieces.  The drive wheels are CD’s with LEGO pieces taped to them to allow for attachment to the axles.  Balloons increase the friction on the cds to prevent spinning. The final design uses florist wire to attach the mousetrap to the chaise.  A K’nex rod is taped to mousetrap as an extension rod.  The final design used the wheels shown, but without the tires.


This mousetrap car was quite capable of going more than 10 m.


After a lot of trials on different flooring surfaces the boys thought they had the right distance figured out.  For some reason their results at the competition didn’t match what they had the day before.


Adjustments to the pull string length seemed the most reliable way of adjusting the travel distance.  A shorter pull string traveled shorter distances.

We tried a braking system, but our design was hard to set and tangled too easily.

I was really pleased that the boys came up with a design that was completely original.  It made the project a lot more fun and interesting.

Using LEGO pieces limited the axle length.  A larger width would have made it easier to earn the centerline bonus, but their design did earn the centerline bonus on one of its two runs.

Almost all the cars at the competition used the same design,  a rectangle of basswood with four cd wheels and threaded axles.  They used a longer extension arm that was pinned down when the mousetrap was set.  The result was they could leave the string attached to the axle and self brake.