Cardboard Catapult Levers

Who doesn’t love the Angry Birds game? With the swipe of you finger on a touch screen you can sling silly little bird creatures toward teetering towers and watch them crumble as you earn precious points. These Angry Bird slingshots are modern day catapults, which are a simple machine called a lever. Catapults were the height of military sophistication back in the medieval age. These contraptions helped warriors to easily throw large heavy rocks at their enemies over great distances. (For background on simple machines, please read our previous post about ways to introduce the idea of simple machines and how you can “upcycle” everyday materials you already have access to.)

Almost any object can become a lever when it is rests upon or rotates around a fulcrum. By placing an object (load or resistance) on one side and applying pressure (force or effort) on the other side, the lever presses against or rotates around the fulcrum and moves the load. There are three classes of levers which depend upon the placement of the fulcrum and force (learn more about levers here on BrainPop). We like to demonstrate the most recognizable lever, a seesaw, using a paint stick stirrer (lever) and an empty toilet paper tube (fulcrum). When the fulcrum is placed in the middle of the lever you can apply gentle force to one side and watch it raise the load from the other side. If the fulcrum is moved away from the middle of the lever it alters the amount of force needed to move the load. If a lot of force is applied it turns our lever into a launcher. You can glue a plastic soda cap to one end of the paint stirrer stick and launch cotton balls or pom-poms into the air to demonstrate. This PBS Kids video is a another quick way to demonstrate a basic lever.

Levers can be even more fun when you create additional force by adding a spring, which is what truly gives a catapult lever its power. There are many types of catapult designs that range in complexity. There is even a group of people who make their own catapults to launch pumpkins each year (watch the Science Channel video here). You can create your own powerful tabletop catapult using a pencil as a lever, a twisted rubber band spring, and a sturdy box as your fulcrum (based on this tutorial by Lorriane at Ikat Bag). Let’s get ready to launch!

Gather Materials:

  • Medium-sized sturdy box for the base
  • Small cardboard box (matchbox, section of egg carton or scrap cardboard to make launching basket)
  • Thick rubber band (needs to be strong enough to be used as a spring)
  • Pencil (or wooden dowel rod)
  • Toothpicks (or wooden dowel rod cut to two pieces about 3″ each)
  • Pom-Poms (the load)
  • Scissors (or xacto knife)
  • Tape (masking tape or duct tape)
  • Optional decorative elements (markers, paint, feathers, etc.)
  • Consider using a yard stick on the floor to measure distance

Make It:

  1. Tape the basket to the end of the pencil. You can use a section of an egg carton, a matchbox, or scrap cardboard. (This is the basket to hold your load and the pencil is the lever or arm of your catapult.)
  2. Stand the box tall and remove one side of your box. (This will allow you to work inside of the box.)
  3. Cut or fold a notch into the top of the box. (Your pencil lever will rest against this notch later.)
  4. Cut a small slit in the middle of both sides of the box. The small slits should line up with the notch you cut/folded into the top of the box. (You will attach your rubber band spring here later.)
  5. Thread the rubber band through the small slits on the two sides of the box. Secure the rubberband with toothpicks or wooden dowels on the outside of the box. Make sure your rubber band is sturdy. If it is too loose it will not work very well. (This will become your rubber band spring that will provide resistance to your pencil lever later.)
  6. Twist the rubberband in the opposite direction that you want your pencil lever to launch. Slide the end of your pencil lever in between the two strands of twisted rubber band.
  7. Hold the box with one hand while you use the other hand to pull the lever down to the table and release. The lever should swing to the top of the box and rest in your cut/folded notch.
  8. Practice launching cotton balls and discuss the variables of the catapult:
    1. Remember that the twisted tension of the rubber band is what generates the force.
    2. The direction that you twist the rubber band is opposite of the direction you want to launch.
    3. The height of the pencil lever impacts your angle and distance.

Activities:

  • Record your observations while launching different materials (i.e. cotton balls, pom-poms, wadded piece of paper). Consider measuring distance, angle, and speed.
  • With two teams, build your own cardboard Medieval castle to protect your catapults and see how many times you can infiltrate the other team’s castle in 1 minute.
  • Use pom-poms to create your own Angry Birds and use them to knock down cardboard box towers.

Design Experiments:

  • What happens if you make a longer lever?
  • What happens if you make a deeper notch in the top of the box?
  • What happens if you use a tighter rubber band?

Want to learn more? Check out these resources: