Engineering Launchers

Students use their scientific knowledge of levers, forces, and energy to design and test a prototype to launch objects. 

• Engineers use what they know about simple machines and energy transfer to design technologies that solve specific problems.  
• Engineers who want to design a technology that can launch objects into the air need to think about how simple machines can redirect force in a way that causes energy to change from potential energy to kinetic energy.
• Catapults can be thought of as systems, and engineers need to think about how the different parts of a catapult work together to redirect forces to launch objects into the air.

• How could thinking like an engineer be useful to solving the problem of launching objects in the air?
• How could a lever be used to design a technology that can launch objects into the air?
• What would engineers need to think about when designing a catapult?
  
  

Misconception: Objects that aren’t moving don’t have energy.

Fact: All matter has energy. Energy can be either stored (called potential energy) or in motion (called kinetic energy). Objects that aren’t moving still have stored potential energy.

Misconception: Each form of energy is created to meet a certain need.

Fact: Energy is never created or destroyed. However, it can change form in an energy system. People can design energy systems such as catapults that change energy from one form to another to do work.

Catapult:  a machine that changes potential energy to kinetic energy so it can launch objects into the air

Effort force: the force you apply to a machine

Launch:  to throw something with a lot of force

Load: an object that a simple machine moves or the force that the simple machine must overcome

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.

In this lesson, students explore the phenomena in kinetic systems by identifying an engineering problem, including its criteria and constraints, that they will then design a technology to solve. Students create a visual model of their prototype before they build it hands-on and test how far their prototype throws balls in the context of an automated kennel. Students evaluate each prototype for effectiveness and possible failure points as they consider ways to improve it.