A lever is basically a motion arm with a load
at one point and a fixed point about which the arm moves, called a fulcrum
. The fulcrum point, the length of the arm, where the force
to move the arm is applied, and the position of the load determine the force generated upon the load and the direction of movement. The greater the ratio
between the distance from the force to the fulcrum and the distance from the load to the fulcrum, the greater the leverage
. In other words, the closer the fulcrum is to the load, the better the lever works (this is true of all lever types.)
A first-class lever is the most basic, and what everyone thinks of when they hear the word "lever". In a first-class lever (ignoring adjectives of quality), the fulcrum is between the load and the force moving the arm. This is the type of lever that Archimedes was referring to when he stated "give me a lever long enough and a place to stand, and I shall move the earth". This type of lever is found in see-saws, pliers, and what a crowbar does.
A second-class lever is one in which the fulcrum is at one end of the arm, the force at the other, and the load in-between. Again, the closer the load is to the fulcrum, the more powerful the lever. This type of lever is what makes a nutcracker (not the puppet kind), a wheelbarrow, and a garlic press work.
An example of a third-class lever can be found close to hand, as there's one in your elbow (badum-bum!) This type of lever has the fulcrum on one end, the load at the other, and the force in between, Almost every load-bearing joint in your body is a third-class lever (with the major exception being the foot, which is a first-class lever.) another well-known third-class lever is the shovel. Spears, rakes, halberds, and other long hand tools can be looked at as both third- and first-class levers as they are used in both ways.