View Full Version : Catapult!
JohnnyEgo
02-04-2021, 20:17
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:
https://www.johnnyego.com/wood/2021Projects/catapult/catapult02.JPG
He crosscut the maple base and frame arms with the sled.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult03.JPG
https://www.johnnyego.com/wood/2021Projects/catapult/catapult04.JPG
Then he cut the finger joints on sled jr.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult11.JPG
I did the single rip cut because it was fairly narrow, then he took over with the walnut for the multi-angle base.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult14.JPG
JohnnyEgo
02-04-2021, 20:17
Drilled holes in the frame for the pivot arm.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult13.JPG
Drilled holes for the angled base.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult17.JPG
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.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult20.JPG
https://www.johnnyego.com/wood/2021Projects/catapult/catapult21.JPG
Two wiped on coats of Danish oil for the win.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult23.JPG
https://www.johnnyego.com/wood/2021Projects/catapult/catapult28.JPG
https://www.johnnyego.com/wood/2021Projects/catapult/catapult26.JPG
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.
https://www.johnnyego.com/wood/2021Projects/catapult/catapult32.JPG
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.
That friggin cool!
Now, on to a full size one for the front yard and capable of launching farm animals.................
I didn't make a catapult until 11th grade physics. A few years ago we picked a science fair project that got the school district, and the state involved, and all the other group member parents bailed their kids out of the project. Nicely done.
That is 1st prize for sure! :D
Awesome project! Looks good too! Let us know how it was received.
JohnnyEgo
02-04-2021, 20:45
I didn't make a catapult until 11th grade physics. A few years ago we picked a science fair project that got the school district, and the state involved, and all the other group member parents bailed their kids out of the project. Nicely done.
That must have been a hell of a project!
To Bear Arms
02-04-2021, 20:51
Dang! Not sound gay or anything, But I always enjoying looking at pictures of your wood projects!! And I thought it would be bigger! But that is a sweet catapult! Glad to see your Son using the power tools and hope he enjoys his time with his Dad in the shop.
Great-Kazoo
02-04-2021, 20:57
He is definitely getting taller. Soon he'll be showing you how to make those projects
JohnnyEgo
02-04-2021, 21:02
Haha I think I could scale it rather easily out of a piece of plywood and some bungees, but we get into enough trouble with the neighbors as it is.
I'd like to believe he will look back on this and enjoy it. Mostly, he considered it a needless distraction that took him away from his video game for an entire Saturday. I've been trying to reinforce the concept of finding satisfaction in the completion of a hard or tedious task well done, even if there isn't any joy in the task itself. He didn't really want to do it, and I had to prod at him a bunch to see it through, but he sure seemed to like being able to say he built it himself, particularly after his friends told him how cool it was.
I recall driving my own father nuts in the exact same way, and while I value the time he spent doing things with me now, I sure resented the hell out of him when he pulled me away from Speed Racer or He Man cartoons. So I'm taking the long view of things.
JohnnyEgo
02-04-2021, 21:03
He is definitely getting taller. Soon he'll be showing you how to make those projects
He asked me if I would buy him a razor yesterday. I have no clue where the time went.
buffalobo
02-04-2021, 21:07
About time you guys built something useful. Desks and furniture are cool but this results in quality time using it after the work and education are done.
#flyingtomatoesthatmomdoesntknowabout
JohnnyDrama
02-04-2021, 21:22
Very cool. Nicely done. It's great that you documented the process. Not only a neat science project but something that is fun to play with. Maybe someday you two can build a much bigger catapult for more serious science.
hollohas
02-04-2021, 21:54
This is AWESOME.
Fentonite
02-04-2021, 21:58
You're a great dad!
^^^
And your son is quite a guy, too!
Rucker61
02-04-2021, 21:59
Damn, he looks so serous in all of those photos. Nice work on project management
Can you show some pictures of how you adjust the angle?
Wolfshoon
02-04-2021, 22:06
Stopsaw for the win! Brother has a 3/16” square notch in the bottom of his thumb bone from a conventional saw. He went out following week and bought a stopsaw.
Great to see a youngster able to properly use tools (instruction is key)
BPTactical
02-04-2021, 22:06
He asked me if I would buy him a razor yesterday. I have no clue where the time went.
He means the Polaris RZR.
Now on to the Trebuchet...
JohnnyEgo
02-04-2021, 23:37
Can you show some pictures of how you adjust the angle?
Pretty simple, really. The pivot arm is also the rotational axis. I had him measure and draw out all the angles between the side plate and the base, and then drill the holes so that two pins would fix it at the desired angle. He did pretty good for most of them, but was a little off on one of the 90 degree holes, which is why the upper front holes have one extra. I told him it was a feature; extra pin holder.
http://www.johnnyego.com/wood/2021Projects/catapult/catapult18.JPG
http://www.johnnyego.com/wood/2021Projects/catapult/catapult19.JPG
http://www.johnnyego.com/wood/2021Projects/catapult/catapult33.JPG
http://www.johnnyego.com/wood/2021Projects/catapult/catapult34.JPG
http://www.johnnyego.com/wood/2021Projects/catapult/catapult35.JPG
I had thought the hardest part of this whole thing would be explaining force vectors and free body diagrams to a 10 year old with zero knowledge of algebra or trigonometry. Turns out the hardest part by far was getting him to sit in a chair long enough to type up what he learned and fix the spelling and grammar errors.
JohnnyEgo
02-04-2021, 23:43
Stopsaw for the win! Brother has a 3/16” square notch in the bottom of his thumb bone from a conventional saw. He went out following week and bought a stopsaw.
Great to see a youngster able to properly use tools (instruction is key)
The most I've been able to give a movie is 1 3/4 thumbs up since 2001, so I was quick to adopt the Sawstop as soon as he grew to table-top height. The other moral of that story is they mean it when they tell you not to use the fence and the miter gauge at the same time in the manual...
GilpinGuy
02-05-2021, 01:49
Build a bigger version and have that dude in Golden toss huge snowballs at the tweaker.
JohnnyEgo
02-05-2021, 08:06
Or just position it under the tree he loves so that it becomes a self-solving problem?
BladesNBarrels
02-05-2021, 09:46
Great project and shop tour.
Best of the tools - SawStop and Festool.
Your son will look back on this someday and realize he was surrounded by the best, including Dad
I see now, the catapult operates the same each time and by changing the floor angle, you're also changing the release angle. That's a good method because it allows the power of each shot to be identical. The one we built in high school just raised or lowered the stop bar, so there was also more or less power behind each shot depending on the stop bar height.
Trigger Time 23
02-05-2021, 13:45
Man, nice shop set -up! Cool project!
That is not a real shop, it is WAY to clean. [LOL]
Love reading these kinds of posts. Yup, in the long run he will look back and smile.
He means the Polaris RZR.
Now on to the Trebuchet...
I'm with BP on the trebuchet. maybe next year!
I am sure your son will appreciate the time with Dad even more down the road! and your shop needs more dust.. more clutter!
Good for you. Teaching great skills and making memories. Can’t wait to see next years project!
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.
This is me. I tried to get my daughter to do a super awesome project, where I'd come up with the experiment, but make her put everything together, and she was never down. I wanted to do a solar powered aquaponics project. She always picked stuff more realistic and appropriate for her age though, so it was for the best.
JohnnyEgo
02-06-2021, 00:07
That is not a real shop, it is WAY to clean. [LOL]
Love reading these kinds of posts. Yup, in the long run he will look back and smile.
Sadly, it's definitely a garage, not a shop. Everything is on wheels so my wife can park her car back in there in the evenings, and she frowns upon piles of sawdust gently raining down every time the garage door opens, so it is kept clean out of necessity and fear.
This is me. I tried to get my daughter to do a super awesome project, where I'd come up with the experiment, but make her put everything together, and she was never down. I wanted to do a solar powered aquaponics project. She always picked stuff more realistic and appropriate for her age though, so it was for the best.
It's an interesting balance. One of my son's friends is doing a bridge experiment, where you design different bridges and see how much weight they will support. I remember a friend of mine doing something very similar way back when we were in 5th grade. The difference is that my son's friend's bridges were clearly designed in Solidworks and accompanied by finite element analysis prints. Even more impressive to me is that the bridges were 3D printed somehow with a color spool such that they have the FEA color map printed on the surface. I am not sure I could do either of these things, let alone a 10 year old. Coincidentally, his dad's a mechanical engineer, but I am sure that has no bearing. I go back and forth over whether I do too much or not enough on this type of thing, but at the end of the day, I still end up frog-marching him towards active involvement in his own education, even when it would be faster, less stressful, and there would be much fewer obscenities uttered by either of us if I did it myself.
My one redeeming factor in trying to help with science fair was that I would struggle with any project just as much as she would, so I wouldn't feel like I was doing it for her.
Aloha_Shooter
02-06-2021, 01:07
Great set of pictures. As a science fair judge, I recommend he keep those pictures in an album or posted on his board to show he did all the work.
Great-Kazoo
02-06-2021, 06:56
Sadly, it's definitely a garage, not a shop. Everything is on wheels so my wife can park her car back in there in the evenings, and she frowns upon piles of sawdust gently raining down every time the garage door opens, so it is kept clean out of necessity and fear.
It's an interesting balance. One of my son's friends is doing a bridge experiment, where you design different bridges and see how much weight they will support. I remember a friend of mine doing something very similar way back when we were in 5th grade. The difference is that my son's friend's bridges were clearly designed in Solidworks and accompanied by finite element analysis prints. Even more impressive to me is that the bridges were 3D printed somehow with a color spool such that they have the FEA color map printed on the surface. I am not sure I could do either of these things, let alone a 10 year old. Coincidentally, his dad's a mechanical engineer, but I am sure that has no bearing. I go back and forth over whether I do too much or not enough on this type of thing, but at the end of the day, I still end up frog-marching him towards active involvement in his own education, even when it would be faster, less stressful, and there would be much fewer obscenities uttered by either of us if I did it myself.
As i'm reading your always interesting post. I flash back to one project in 9th? grade. Myself and a friend since kindergarten, did what we thought was not only factual but humorous project.
Titled The Appian Way, 500 years later.
Very detailed and lifelike of how it looked in present times. Many hours spent laying out the details and finally the finished project. 4"w x 12" long row of 1/2" broken tiles stuck on a board, held together with clay. Then a handful of dirt over it.
Clearly the teacher had no imagination, as we barely got past a c+ for effort. Even then, the education system was rigged.
Next level. Note the blanket at 13:45 for speed testing.
https://www.youtube.com/watch?v=P_6my53IlxY
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