• Sara Thompson

Catapults-Ready for Launch

By Sara Thompson

Special to the Enterprise


F/A-18 ready for catapult launch from the deck of the USS Abraham Lincoln while in Hawaiian waters. Photo by Eric Guinther, November 2004

There are several varieties of catapults throughout history, but all use similar concepts of physics to get the machine to work.  The three main types of “catapults” are, the catapult, a ballista, and a trebuchet.  All three work in different ways, but all serve the purpose of moving an object a greater distance than one could throw naturally.

One of the first types of catapults is simply called a catapult and is what most people think of when they think of catapults.  It has a wide base and a long arm that is pulled back that is then released to move a heavy object.  It works by putting an object at the end of an arm, then pulling the arm back and holding it with a rope, storing potential energy through tension in another rope or spring, once released the arm swing up and forward, launching the object forward.  This method was historically used to launch heavy rocks towards enemies, but more recently can be used to aid in launching airplanes into the air when there is not a long enough runway to gain speed.  School ages students can learn a lot by building and experimenting with catapults.  They can learn about potential and kinetic energies and how they are used, air resistance and gravity, and how to adjust and calculate a catapult to hit a target.

A ballista uses a similar concept to the previous catapult as it uses tension and stored potential energy to work.  With a similar look to a crossbow, a ballista uses ropes and tension by pulling back a projectile.  Once released, the projectile flies forward as the tension is released.  Historically a ballista projectile would have a pointed tip to penetrate a wall or door and be hard to remove.  Modern slingshots use the same conapts as a ballista, using a long rubber band to pull an object backwards, creating tension, then releasing the object forward.

Trebuchets work a little bit different than the other two.  It uses an arm like the classic catapult, but it used a counterweight to swing the arm instead of stored energy.  When the arm is pulled back and loaded with an object, it can be help in place with a rope or other stopper, but when released it is swung up due to a heavy weight pulling the arm from the other side.  Trebuchets are more accurate than other types of catapults and is easier to adjusted by changing how heavy the counterweight is.  Trebuchets are still used today for entertainment purposes, often used in fall to throw pumpkins and compete with others on who can throw it the farthest, or to hit a particular target.

Campers who attend our Everyday Engineering Fall Camps will learn how to build a catapult as well as many other building projects that includes racetracks, boats, parachutes, and buildings.  

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