How To Make A Catapult

Did you miss the previous post on How To Work With Cardboard? I've been having some trouble getting my feeds out recently. New posts are published on the blog itself but aren't being picked up by the aggregators. Often, I find that I need to send out a new post before the old one gets picked up in tandem. Anyone having a similar problem? 

Anyway, today we're going to build a cardboard catapult. This is one of those weapons commonly seen in, say, medieval times, whenever there was a siege on a fortress or castle. The original design was from an out-of-print book on papercraft I have at home.

What you need:

• Sturdy box (does not need to have a lid)
• Thick, strong rubberband
• Pencil or short dowel
• Matchbox, or small shallow box
• Toothpicks

in addition to the usual tools and things for working with cardboard: masking tape, scissors, craft knife.


Step 1
Make the basket. You can use a matchbox, or make your own basket out of a piece of cardboard as shown:

Step 2

Tape the basket onto one end of the pencil. This is the lever/arm of the catapult. Set aside.

Step 3

If the box has a lid, remove it.

Stand it up tall as shown.

Make two cuts in the top of the box, beginning with the open face, and cutting half way to the other side (this will be clearer in the next step).

Step 4

Fold down the flap released by the two cuts you made in Step 3. You may need to score the cardboard first if it is thick.

Step 5

Make a hole in the side of the box, about half-way down, and just under the fold you made in Step 4.

Repeat on the other side of the box to make a second hole.

Step 6

From the inside of the box, thread the rubber band through one of the holes 

and push a bunch of toothpicks through the loop. Pull on the rubber band from the inside of the box to secure the toothpicks in place.

Step 7

Pull the rubber band through the other hole and secure externally with toothpicks. Note that the rubber band needs to be extremely taut for the catapult to work. This means that if you managed to do Step 7 without struggling, your rubber band is too loose. My rubber band was too short to even reach the other hole (which is how I knew it was a good one), so I looped some string through it, and pulled that through the hole from the outside, to draw the rubber band out with it.

Step 8

Begin twisting the middle of the rubber band in the direction that's opposite to how the arm of the catapult will swing to launch its load.

Step 9

Separate the two strands of the rubber band (still twisted) and push the other end of the pencil through it. 

Release the lever/arm and it should swing up and hit the top of the box. 

Troubleshooting for the lever/arm:

• If it swings downwards, your rubber band was twisted in the wrong direction. Remove the pencil and retwist the rubber band before re-inserting the pencil.
• If it swings too slowly or not at all, your rubber band was too loose/feeble/thin/long. Replace it a thicker, shorter one.
• If the lever/arm is stuck, readjust the depth of insertion of the pencil through the rubber band. It might be hitting the inside of the box.

To launch, load the basket

and release!

With a really good rubber band, the force of the launch may tip the entire base over. So hold on to it!

For older kids, this is a useful visual aid for a Physics lesson in kinematics and energy. For instance:

• Demonstrate projectile motion - change the angle of the arm/lever at the launch moment by using different depth cuts in Step 3. Discuss the resultant range, angle, height and force of impact of the projectile.
• Demonstrate energy transformation - elastic and gravitational potential energy (PE) to kinetic energy (KE) to sound energy.
• Build the catapult in different sizes, or with different rubber bands, or pencils of different length - discuss the changes each variable makes to the resultant motion of the projectile. 
• Demonstrate circular motion and rotational KE.
• Based on the relevant principles, have the kids design and build other working models of catapults. Here are some ideas.

With younger kids, though, it's just a fun toy. Build a cardboard castle, load up the battlements with Playmobil, Legomen or Princesses and re-enact your favorite stories. Enjoy!