Wikijunior:How Things Work/Lever
A lever is an object that is used with a pivot point, or 'fulcrum', to multiply the force applied to another object. Levers are often long and skinny and made of rigid material. Levers are one of the six simple machines.
Who invented it? 
Levers occur in nature. In fact, your arm and your jaw are both examples of levers. It is impossible to say who invented the first mechanical lever. Human beings have used mechanical levers since the stone age.
The earliest remaining writings regarding levers date from the 3rd century BC and were provided by Archimedes. "Give me the place to stand, and I shall move the earth." is a remark of Archimedes who was the first to mathematically describe how levers multiply force.
In ancient Egypt, builders used the lever to move and uplift obelisks weighting more than 100 tons. A shadoof is type of lever that was used in Egypt. It is a pole with a weight on one end. It is used to lift water from a well or river for irrigation. It was in common use by 2000 BC. It is still used in many areas of Africa and Asia to draw water.
What does it do? 
A lever enables people to do work using less force. A lever usually is used to move or lift objects. Sometimes it is used to push against objects, but not actually move them. Levers can be used to exert a large force over a small distance at one end by exerting only a small force over a greater distance at the other.
How does it get power? 
Its power comes from outside forces acting on it. The force applied to one end of the lever is transferred to the other end on the lever. The real power of levers comes from a mechanical advantage. The lever allows less effort to be expended when moving an object, but there's a trade-off. The object will move a shorter distance than the person or thing exerting the force. For example, a crowbar can be used to pry up a board because it applies a lot of force over a short distance when the person using the crowbar uses much less force over a much greater distance.
How does it work? 
The lever has two important parts. The lever itself and the fulcrum. The placement of the fulcrum determines how far the levered object will move, and how much force is required to move it.
If a weight was resting on a lever a person could lift the weight by pressing on the lever on the other side. The farther away from the fulcrum that person pressed, the less force that person would need to apply. In order to lift the weight the same distance, the force would have to be applied over a longer distance. In science, we call how much effort it takes to move something a certain distance "work." With a lever, you always do the same amount of work no matter how long your lever is. But if you are moving the lever further, then you don't have to push as hard to do the same amount of work.
How dangerous is it? 
Levers can be dangerous because they multiply force. But everyone uses levers all the time without ever thinking they are dangerous. Every time you open a door, you are using a lever. Most levers we use are safe. But throughout human history, levers have also been used as weapons. Some examples of weapons that are levers are nunchucks, catapults, and atlatls.
How does it vary? 
There are actually three types of levers! They are called first-class levers, second-class levers, and third-class levers.
A see-saw is an example of a first-class lever. A first-class lever the fulcrum is located between the force pushing down- the input force-(on a see-saw that would be the person going down) and the output force (the person going up).
A wheel barrow is an example of a second-class lever. In a second class lever, the resistance is located between the effort and the fulcrum. The input force would be the handles, where you need to pull up in order to lift the weight in the barrow. The fulcrum located on the axle of the front wheel.
A baseball bat is an example of a third-class lever. In a third class lever the effort is between the fulcrum and the resistance. In the case of a baseball bat, you exert effort by swinging the bat at the handle. The heavier part of the bat is the resistance. In this case, the force at the end of the bat is actually reduced, but the speed is increased. The end of the bat moves faster than the grip where the force is applied, giving it greater momentum, causing it to strike the ball harder.
How has it changed the world? 
The lever has changed the way we work immensely. It has helped us to do more work with fewer resources. It played an important role in developing agriculture. It also played a crucial role in building shelters and other types of buildings. In fact, levers are so useful and common, that they have played a role in every other technological advancement.
What idea(s) and/or inventions had to be developed before it could be created? 
Levers are one of the six simple machines. They did not require any prior invention. The very first machine ever invented by a human being may very well have been a lever.