Catapult!

[JohnnyEgo]

My son is in 5th grade, which means it's science fair time in this county. After ruling out all the ideas that were improbable if not impossible, such as the robot butler and the hoverboard, and the things I was not willing to do, like the paper mache volcano or winding 40 feet of copper around a motor armeture, we stumbled across ballistic trajectory as a topic. Which totally called for catapults.
Had my son draw up a design first:

Then grab some scrap out of the bins:


He crosscut the maple base and frame arms with the sled.



Then he cut the finger joints on sled jr.


I did the single rip cut because it was fairly narrow, then he took over with the walnut for the multi-angle base.


Drilled holes in the frame for the pivot arm.


Drilled holes for the angled base.


I trimmed up the lever arm on the bandsaw. I won't let him anywhere near the router, so I also knocked the edges off with a roundover bit. Then back to him for glue and clamps.




Two wiped on coats of Danish oil for the win.





We originally used 4 rubber bands, which ended up launching the ping pong balls something like 20 feet. I down-powered it so we could run the experiments at a reasonable scale in the garage.


We used video to determine the horizontal distance and elapsed time. In the context of his algebra-less experiment, that was enough to prove which launch angle sent the ball the furthest. I showed him how to use the kinematic equations to resolve vertical displacement and the component vectors of velocity, but I wouldn't say he was highly interested in that. But we had fun making the catapult and shooting ping pong balls all over the place. And when Mom wasn't looking, we launched cherry tomatoes all over the kitchen as well.

I am proud of the fact that his project represents both his actual levels of understanding and effort, and I look forward to seeing it between the projects of the kids who home-sequenced the genome of their dog and the one who built a functional model of the Jarvik heart.

[Aram]

What a great father-son project! So... uh... how much would he want for one and what is the lead time?

[lift mechanic]

Great to see the younger generation getting interested in woodworking. A great father and son project. Your are a lucky man.

[JohnnyEgo]

Well thank you both!

It's a mixed interest in the sense that he doesn't really like to be pulled away from video games, and wasn't real happy when I made him spend a Saturday working on it. I reminded him I passed 5th grade science already, so it wasn't my job to build it for him. But he liked it a lot once it was built, and he seemed pretty proud to brag to his little friends that he built it when they told him how cool it was. I've been able to lure him into the shop a couple of times in the past two years with varying degrees of attention and interest, and once I got him going he seemed pretty into it, so I am hopeful.

I don't fault him in the sense that I did the same thing to my Dad when I was his age. I really appreciate all the time my father spent with me now, but I was mostly bitter that he pulled me away from Speed Racer and He Man cartoons then. So I am taking the long view.

[Bill Wilson]

I reminded him I passed 5th grade science already, so it wasn't my job to build it for him.  

I passed 5th grade science too, but I don't know if I could have designed that.  


Waaaay cool!  Great job by both of you.

[fredhargis]

That is so cool! Please let us know how it's received at the science fair.

[TomFromStLouis]

Way cool. Next year step up to a trebuchet and those cherry tomatoes will be leaving red spots all over the kitchen, or the neighbor's roof.

[mike9]

Great project, I love seeing kids in the shop making fun projects. I always try to get my son into the shop so he starts to get some interest in woodworking.

[Martin S.]

Is your son still in Scouts? I would like to see him make Eagle and go to Philmont.

[Turner52]

Wonderful project. Something you both will remember forever