The Lever

A lever is one of the simplest mechanical devices.  A lever consists of a beam or stick or rod. However, a lever by itself is not effective.  It must have something on which to pivot.  This pivot is called a fulcrum.  A lever helps to lift weights with less effort. Children on opposite ends of a seesaw lift each other's weight up and down easily.  Anyone who has ever pried something loose with a crowbar or a board has used a lever.

Classes of Levers

First-class levers have the fulcrum placed between the load and the effort, as in the seesaw, crowbar, and balance scale. If the two arms of the lever are of equal length, as with the balance scale, the effort must be equal to the load. If the effort arm is longer than the load arm, as in the crowbar, the effort travels farther than the load and is less than the load.

Second-class levers have the load between the effort and the fulcrum. A wheelbarrow is a second-class lever.  The wheel’s axle is the fulcrum, the handles take the effort, and the load is placed between them.  The effort always travels a greater distance and is less than the load.

Third-class levers have the effort placed between the load and the fulcrum. The effort always travels a shorter distance and must be greater than the load.  A hammer acts as a third-class lever when it is used to drive in a nail: the fulcrum is the wrist, the effort is applied through the hand, and the load is the resistance of the wood.  Another example of a third-class lever is the human forearm: the fulcrum is the elbow, the effort is applied by the biceps muscle, and the load is in the hand.

The Law of Equilibrium

A lever is in equilibrium when the effort and the load balance each other. The law of equilibrium is: The effort multiplied by its distance from the fulcrum equals the load multiplied by its distance from the fulcrum. This law of equilibrium is true for all classes of levers.

For example, 2 pounds of effort exerted 4 feet from the fulcrum will lift 8 pounds located 1 foot on the other side of fulcrum, as shown below.  This effort times distance about the fulcrum is the torque, the rotational force referred to in the gears (wheel and axle) tutorial.