If you have ever played on a see-saw or removed a nail from a wall using the claw on a hammer, you have seen a lever at work. A lever is a simple machine used to decrease the effort required to move objects. It consists of a board or bar positioned on a pivot point, called the fulcrum. A lever is used to lift a load using a force of effort applied at a second point along the board. In order for a lever to work, a force (either a push or a pull) must be applied to it. The amount of weight that can be lifted using a lever varies depending on the position of the fulcrum and the length of the board.

Not all levers are the same. There are three classes of levers which differ depending on the position of the fulcrum in relation to the load. In Class 1 levers, the fulcrum lies between the load and the effort. When Class 1 levers are used, a downward force causes the load to move up, while applying an upward force moves the load down. Common Class 1 levers include seesaws, crowbars, and balance scales. Some common tools are made up of multiple levers. For example, pliers and scissors are each made up of two Class 1 levers.

In Class 2 levers, such as a wheelbarrow, the load lies between the effort and the fulcrum. With a Class 2 lever, the direction of the effort is also the direction that that the load will move. So, in the case of the wheelbarrow, the wheel acts as the fulcrum and the effort is exerted at the handles, so when you pull up on the handles, the materials in the wheelbarrow move upward as well. A diving board, bottle opener, and the claw part of a hammer are examples of Class 2 levers.

In the case of Class 3 levers, the effort is placed between the load and the fulcrum. A hammer, when used to force a nail into something, is one example of a Class 3 lever. The wrist acts as the fulcrum while the load is the resistance of the material into which you are driving the nail. When a Class three lever is used, the load moves in the same direction as the force. Tweezers are an example of an object made up of two Class 3 levers. When you push the tweezers together at a point in the middle of the tweezers, the two tips are also pushed together, allowing you to grab onto a small object. Class 3 levers are also used when you want to increase the speed at which a load moves. A baseball bat and a golf club are examples of Class 3 levers. This allows the hitting end to move faster than your arm.

With all levers, if the effort is fixed, applying that effort at a location further away from the fulcrum will result in an increased amount of force. This is why a longer board is able to lift more than a shorter board. And we can say that the force is directly proportional to the length of the lever arm. More practically, this law can be applied when an adult and a child try to ride a see-saw. In order for the two sides to balance, the adult will have to sit closer to the fulcrum. How much closer? That will depend on the ratio of the adult's weight to that of the child.

To learn more about levers, check out Levers: Raising the Moai on Easter Island and Energy Transfer in a Trebuchet.

To learn more about simple machines, check out Building Simple Machines: Plant Quencher.

To learn more about using models, check out Testing with Models.

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