DIY Catapults Design and build your own catapults with some simple craft supplies!

Catapults have been used for centuries throughout history by armies to launch at their enemies. They were first used in and India, and developed in an attempt to increase the range of . The and catapult use similar principles: taking stored potential energy built up in the form of springs, twisted rope, or any elastic material, and using its sudden release to propel a payload. They were particularly useful during the of medieval and walled cities in the Middle Ages, until people developed such good defenses against catapults that they fell largely out of use in military attacks (source: Wikipedia).

A quick physics lesson before we get into catapults again: Potential energy is any kind of energy that can be stored. You are already familiar with electrical potential energy, in the form of batteries. Gravitational potential energy is what makes objects fall: a ball at the top of the hill has a lot of gravitational potential energy since it wants to roll downhill, for example. Elastic, or spring potential energy is energy stored as a result of deformation of an elastic material such as a metal spring or rubber band. When you hold a stretched rubber band, it wants to snap back to its original state (sometimes painfully! Ouch!). These forms of potential energy can be released in the form of movement; that is, potential energy is converted into kinetic energy. Gravitational potential energy stored by the ball at the top of the hill is converted into kinetic energy as the ball moves downhill.

There are two basic types of catapults, the and the , and each uses a different form of conserved energy to launch projectiles (source: https://scienceoxford.com/wp-content/uploads/2016/03/3.-Catapult-.pdf​).

The Mangonel is probably what you think about when you think “catapult”: you wind back the throwing arm, and release the stored energy to fling the arm forward and launch the . Think of launching an elastic band off your finger: you pull back the elastic, which stores energy in the material since it wants to return to its unstretched state, then you let go, and all that stored energy is released as movement, sending the elastic band flying across the room as the elastic goes back to its original shape. The throwing arm of a Mangonel catapult is bent back over a pivot point (P in the diagram) and restrained by a rope until loaded and aimed, then ​ ​ the restraining rope is released to rotate the arm quickly back over the pivot. This is elastic potential energy in action.

The trebuchet was more common in ancient times, and uses a counterweight to rotate the throwing arm instead of the elastic-like mechanism in the Mangonel. The arm in its unloaded state is already rotated back, with some kind of basket on the other end to hold the counterweight. As the heavy counterweight falls, the energy stored as gravitational potential energy is converted into the kinetic energy of the projectile, flinging it into the air.

The activity: Design your own catapult and build it using whatever you can find in your house!

Start by drawing out your ideas on a sheet of paper. Play around with different-looking frames or bases to keep your catapult steady while launching. Experiment with both kinds of catapults, too. Make sure to label all your parts and what materials you think you’ll need.

Stuck for ideas? Try looking up some ideas on Google! Or, here are some examples of homemade catapults:

Once you’ve got a solid plan, it’s time to start building! Here are some example materials to get you started. Feel free to use whatever else you think will help you build the most accurate and far-throwing catapult! ● Popsicle sticks (experiment with different sizes!) ● Wooden dowels/chopsticks ● Rubber bands (experiment with different thicknesses and sizes!) ● spoons ● Glue (hot glue, liquid glue, whatever works!) ● Tape Once you’ve built your catapult, it’s time to test it! Some projectile you could use are pom-poms, erasers, marshmallows, or even just crumpled up balls of paper. Make sure you only launch your catapult in an open space so you don’t accidentally hit anyone or anything.

Here are some things to think about when testing: Did your catapult work? Did it break when the throwing arm snapped forward? Was the base sturdy enough, or did the catapult move from the force of the launch? What could you do to make it launch farther? Is it precise? If you launch five projectiles in a row, do they all land in roughly the same place, or does it change every time? Is it accurate? Do the projectiles land where you aim the catapult?

Take what you observed during your tests and try and improve your catapult. Maybe you need to add more supports to the base. Try changing the length of your throwing arm, or the amount of counterweight you use (in the case of the trebuchet). What about a thicker rubber band?

Test again once you’ve made some changes, and observe the effects these changes had. If you still think your catapult could be better, go back and tweak some things again. This process of designing, testing, observing, and redesigning is what engineers do in the field all the time. Now you’re thinking like an engineer!