Why Build a Catapult?
If you're a guy, this probably seems like a strange question. There's something in the male mind that just loves to see heavy objects flying through the air, and if they're being fired from something you've made then that's all the better. But there are loads of reasons why building a catapult with your children is a great idea, whether it's for a school project or just the coolness factor.
One of the very best reasons is that it can help give a frame of reference for the physics concepts that your children are being taught in school. A common illustration used when considering forces is that of a child throwing a ball and the forces which act upon it.
Gravity pulls the ball toward the ground in a parabolic arc, while air resistance slows it. When designing your catapult you could try asking your child how far they think it will be able to throw, or perform experiments to see if changing certain factors, like the object you are shooting, makes for a better fling.
It's a sad fact that in the modern schooling system the time and budget allocated to letting children experiment with hands-on engineering projects is being heavily cut. A project like this is a great way to spend time with your child outside of the normal authoritative parental role, allowing them to experience things that the school system is increasingly putting aside in favor of more book-based teaching.
Appropriate Catapult Resources By Age Level
When considering any project with your child, it's important to take into account his or her age. While teenagers might love the idea of making a six-foot tall trebuchet, it's probably not appropriate for a six year old. Below are a couple of examples of projects, along with guidance on what might be an appropriate age range.
Ages 8 and Below: For this kind of age range, it's probably a good idea to make quick, simple and above all low-powered catapults. Designs based around pegs or mouse traps are probably ideal, and should be easy to make in a short amount of time with basic household materials.
Pre-Teens: Now that your children can (presumably) be trusted not to break windows, it's time to investigate slightly larger models of catapult. Amazon sells several kit form catapults, but table top catapults can be built quite easily using wood and rubber bands.
Teenagers: Time for the big plans: For a single teenager, building a mangonel might be ideal. These plans from the Open University require wood-working tools and a work-bench, but make a fantastic ratchet based catapult. Other projects, ideal for groups of teenagers, make use of PVC piping to create life-sized catapults. If you're a leader of a scout troop or similar youth organization, having a contest to see who can build the best catapult might be an interesting weekend project. Of course, you may just decide to use them for defending farmland from local vandals instead.
Hopefully this has given you some ideas of where to start when building a catapult, and some guidance on why it's such a great idea. If you're interested in doing other projects of a similar nature to this I would highly recommend Storm the Castle, which has an inventive array of user-submitted and commercial ideas for siege engines of all types.