Students use their scientific knowledge of levers, forces, and energy to design and test a prototype to launch objects.
Prepared hands-on materials, full year grade-specific curriculum, and personalized live professional development designed to support mastery of current state science standards.
Catapults Launch Objects
A catapult is a machine that changes potential energy to kinetic energy so it can launch objects into the air. To launch means to throw something with a lot of force.
Remember that energy can either be stored or in motion. Energy that is stored is called potential energy. The energy of motion is called kinetic energy.
There are different forms of potential and kinetic energy. Chemical energy is one form of potential energy. It is stored in the bonds that hold together atoms and molecules. All moving objects, from a moving car to a running dog, have kinetic energy.
Catapults can use different forms of potential energy. For example, some fishermen use catapults to launch bait into the water. Bait is used to attract the fish they want to catch.
The catapults used by fishermen are simple. They are made up of a kind of rubber band. This rubber band stores elastic energy. Elastic energy is a form of potential energy. It is stored in objects when they’re stretched. The more the rubber band is stretched, the more elastic potential energy it stores.
In the catapult used in fishing, the bait is placed at the end of the rubber band. When the rubber band is released, the elastic potential energy changes to kinetic energy. This energy launches the bait into the water.
A Brief History of Catapults
People have used catapults since the time of the ancient Greeks. One common ancient catapult was called a trebuchet. A trebuchet used a lever and a sling to throw heavy stones at enemy castles or walls.
The trebuchet moves in a similar way to a seesaw. One end moves up as the other end moves down.However, there are some differences. The first difference is that the fulcrum isn’t halfway between the effort force and the load, as it is in a seesaw. Instead, the fulcrum in a trebuchet is located closer to the side where the effort force will be applied. This makes one side of the lever arm shorter than the other. There is a heavy weight called a counterweight. It is attached to the short arm of the lever.
The load is the object to be launched. It is on the opposite side of the fulcrum. It is usually placed in a sling at the very end of the long arm of the lever.
Energy Changes in a Trebuchet
Trebuchets launch heavy objects like rocks because they change gravitational potential energy into kinetic energy.
This gravitational potential energy comes from the counterweight. When the counterweight is pulled up off of the ground, it gains gravitational potential energy. The higher off the ground it is, the more energy it stores. Also the more massive it is, the more potential energy it has.
When the counterweight is dropped to the ground, gravity pulls down on it. This causes the gravitational potential energy to change into kinetic energy. This kinetic energy pulls down on the lever. This causes the long arm to move up as the short arm is pulled to the ground. As the long arm moves up, it launches the sling holding the load.
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