Ancient Roman Torsion-Spring Catapult Onager

60 mins Ages 10-14 Indoor

A challenging, hands-on discovery and STEM project designed for curious young minds aged 10 to 14.

Materials Needed

  • • Jumbo wooden craft sticks (18 sticks)
  • • Fast-drying wood glue or hot glue (1 set)
  • • Braided nylon cord (paracord or mason line) (1.5 meters)
  • • Wooden throwing arm with plastic bottle-cap cup (1 unit)
  • • Soft modeling clay or mini marshmallows (1 small ball)

Pro Tips

  • → The Roman onager was nicknamed "the wild ass" because of its violent upward kick when the throwing arm slammed into the cross-stop.
  • → Twist a vertical loop of nylon paracord through holes in the wooden chassis using dowels to store mechanical energy in rope torsion.
  • → Pull the arm back against torsion resistance, place a projectile in the cup, and release: hitting the crossbar releases the projectile.

Safety First

Launch only soft clay or mini marshmallows; never aim at people or fragile items.

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Step-by-Step Instructions

1

Step 1

Build a rectangular box chassis from glued wooden craft sticks, adding a padded horizontal cross-stop bar.

2

Step 2

Thread a loop of braided nylon cord through the sides and insert a wooden throwing arm into the center of the bundle.

3

Step 3

Twist the cord bundle using dowels on both sides to build torsional spring tension until the arm rests firmly against the cross-stop.

4

Step 4

Pull the arm back flat, load a soft clay ball into the cup, release, and measure flight distance.

What they are learning

Torsional potential energy storage Hooke’s law in cord skeins Abrupt arm stop trajectory physics (Onager kicking motion)