# We Teach You How to Build a Catapult in the New Popular Mechanics Kids' Book

© Philip Friedman Step-by-step plans for an all-ages version of the Roman-Empire siege engine.
© popularmechanics.com 100+ STEAM Activities Kids Won

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For over 100 more activities like this, Popular Mechanics’ new book 100+ STEAM Activities Kids Won’t Learn In School has the ultimate collection of experiments, projects, riddles, and trivia designed to help kids master early concepts in Science, Technology, Engineering, Art and Math. Buy it now and start your kid’s journey to making something great!

The Roman empire’s onager catapult used torsion—twisting an object, in this case a restraining rope or spring, by applying torque—to build tension in the arm and generate a powerful swing. The onager catapult was a central siege engine for the Romans from the 4th century AD until the 6th century AD, capable of launching projectiles over 400 feet to breach castle doors and walls. Our design can’t quite do that, but it’s perfect for kids to build and use in games and experiments.

Building this catapult takes math, science, and engineering skills. Precise measurements are key to making all the pieces work well together. Let’s understand the science behind mechanical energy and torsion, and put that maker grit to work!

© Alex Griendling parts and supplies list

## Make the Catapult Frame

Drill four 1/16-inch pilot holes through the 10-inch frame pieces, with the holes 1¾ inches from each of the four ends and centered vertically. Drill a pilot hole into the center of each end of the crossmembers [1].

Apply glue to the ends of one crossmember and near the pilot holes on the inside of the 10-inch frame pieces; screw the crossmember into place [2]. Repeat this step on the other end.

Drill 3⁄16- and ¼-inch holes through the sides of the frame, 5 inches and 5½ inches on center, respectively, from the front of the frame. Glue the footings to the bottom of the frame, flush with the sides and 1 inch on center from the ends [3].

Drill pilot holes into the center of the ends of the footings; screw the wheels into place [4].

## Install the Uprights and Upright Supports

Using the drill-glue-and-screw process, secure the uprights to the frame [5]. The uprights should be 4¼ inches on center from the end, the screws driven in from beneath the frame. Drill, glue, and screw a crossbeam flush with the tops of the uprights. Drill, glue, and screw the supports into position [6].

## Make the Throwing Arm

Drill a 1⁄8-inch hole through the center of the long dowel about ¾ inch from the end [7]. Use the round face of the hammer to shape the washer into a cup [8].Attach the washer to the dowel (the cup’s rim faces up) using the bolt, lock washer, and nut [9].

## Assemble the Torsion Spring

Fold the nylon cord in half twice; tie the loose ends with a square knot [10].

Feed the folded cord through the ¼-inch holes [11], leaving ½ inch of cord on the outside of the frame. Place a washer over each bundle of cord .

Insert the 5⁄8-inch dowels into the cord loops [12] ; position the throwing arm in the middle of the cord.

Rotate the dowels toward the front of the frame until the arm presses against the crossbeam [13]. Insert the 1⁄8-inch dowel into the smaller hole.

Lower the throwing arm down to the frame. Mark spots on the frame and arm where the anchor and eyebolt will be screwed in. Increase the cord’s tension by turning the dowels forward. Lower the arm, hook it into position [14], and remove the locks.

To fire the catapult, load the bucket with a projectile and carefully push the anchor hook out of the eyebolt [15].

### TAKE IT ONE STEP FURTHER

This catapult is on wheels. Calculate the distance versus speed while moving it to achieve the trajectory you desire. For example, if you are under attack, do you move your catapult backward as the enemy draws closer? Do you angle it to force your projectiles toward a specific target? Do you launch while moving? How does that affect the trajectory of the projectiles? Have fun experimenting!

© . pop projects POP Projects is a collection of new and classic projects from more than a century of Popular Mechanics. Master skills, get tool recommendations, and, most importantly, build something of your very own.

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