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Trebuchet essay

In this video segment adapted from NOVA, a team of historians, engineers, and trade experts recreate a medieval throwing machine called a trebuchet. To launch a projectile, a trebuchet utilizes the transfer of gravitational potential energy into kinetic energy. A massive counterweight at one end of a lever falls because of gravity, causing the other end of the lever to rise and release a projectile from a sling. As part of their design process, the engineers use models to help evaluate how well their designs will work.

This video is available in both English and Spanish audio, along with corresponding closed captions.

In medieval times, one of the most fearsome weapons was a trebuchet—a powerful machine used to hurl projectiles. Combining the launching capabilities of a catapult and a sling, trebuchets were able to throw massive objects at high speeds and over great distances. Trebuchets were also more accurate than a basic catapult and more powerful than a simple sling; they could throw objects hundreds of pounds in weight to targets hundreds of yards away. During a siege, a trebuchet could be used to crumble castle walls by launching large rocks or to spread disease to the people inside the castle walls by tossing over dead animals.

Imagine what would happen on a seesaw (a simple lever) if a book were placed on one end and you were to jump on the other end—the book would go flying into the air. A trebuchet functions similarly, except that the pivot point of the lever is placed off-center (in this case, farther away from the load) to amplify the force that launches the projectile. The load, or object to be moved, is placed in a sling (composed of a pouch and two cords) attached to the long arm of the lever. A very heavy counterweight is attached to the short arm of the lever. When the counterweight falls, the long arm of the lever is raised very quickly into the air, pulling the sling up with it. As the sling swings into a vertical position, one end of the sling is unleashed, opening up the sling and releasing the load at a high velocity.

To put a trebuchet into its "cocked" position, a team of people use their energy to hoist the counterweight. When the counterweight is raised, it gains this energy as potential energy. In this case, the potential energy is gravitational potential energy—energy that results from the position of an object in a gravitational field. The amount of gravitational potential energy is dependent on both the weight of the object and its position, or height, above the ground. Since the counterweight in a trebuchet is very heavy, it has a great deal of potential energy.

While energy cannot be created or destroyed, it can change forms. The potential energy from the raised counterweight is transformed into kinetic energy—the energy of motion—as the counterweight is released and begins to fall. The downward motion of the counterweight then causes the sling to swing and the projectile to be released. The potential energy that was stored in the raised counterweight is transferred, in the form of kinetic energy, to the projectile, which is released at a high velocity.

Design a working model of a trebuchet and demonstrate the power of a Class 1 lever in this NOVA classroom activity .

  • Where does the potential energy in the lever arm come from?
  • Where is there potential energy throughout the loading, cocking, and releasing of the trebuchet? Where is there kinetic energy?
  • What evidence is there that energy is conserved?

Use these questions to create a simple assessment or video-based assignment with the Lesson Builder tool on PBS LearningMedia.

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