Popular Mechanics - USA (2022-05 & 2022-06)

he 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 A.D. until the

6th century, 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

precise measurements, skill with a drill, and

an understanding of how the pieces work

together to launch a projectile (so read the

instructions first). It’s a fun, hands-on lesson

in mechanical energy and torsion—science!

PARTS AND SUPPLIES

One No. 10 bolt, ¾ inch,

with nut and lock washer

One 36-inch length of

18-pound nylon cord

Two ¾-inch washers,

¾-inch outside diameter,

¼-inch inside diameter

Two 5⁄8-inch-diameter

dowels, 1½ inches long

One 5⁄8-inch-diameter

dowel, 1½ inches long

One small eyebolt and

anchor, one screw eye

• Manual saw and miter box


• Ball-peen hammer


• Yellow wood glue


• 18 No. 6 wood screws,
  1 ½ inches long


• Power drill with 1⁄16-, 1⁄8-,
  and ¼-inch bits

1

Two pine or ash 1x1s cut into

trapezoids with 45-degree

angles at the ends; 4 inches long at

top (upright supports)

Tw o 1 x 1 s ,

10 inches long

(frame)

One 5⁄8-inch-

diameter dowel,

8½ inches long

One fender

washer, 2-inch

outside diameter,

¼-inch inside

diameter

Ammo (marbles

work best; just

avoid sharp objects)

Two 1x1s,

6 inches long

(footings)

Five 1x1s, 4 inches

long (uprights,

crossmembers)

Four pine-wood

derby car wheels,

2-inch outside

diameter

5

2

3

4

6

1

4½ inches

5½ inches

2

3

4

5

6

64 May/June 2022 ILLUSTRATION BY ALEX GRIENDLING