A lever is a rigid bar that turns or pivots
around a fixed point called a fulcrum.
A First class lever is a lever that has the fulcrum in-between the effort and the resistance. In a first class lever, the farther one applies force from the fulcrum, the easier it is to lift up objects. The closer one gets to the fulcrum, the more effort is needed to lift up the resistance. In addition, the resistance always goes in the opposite direction of the effort.
A second class lever is a lever in which the resistance is located between the fulcrum and the effort. A second class lever always increases the effort force. An example of a second-class lever is a wheelbarrow. The fulcrum is the wheel and the resistance in a wheelbarrow is the object being moved while the person pushing the barrow supplies the effort. The resistance arm is the distance from the fulcrum to the object.
a third-class lever, the effort is between the resistance and the fulcrum.
A third-class lever always decreases the effort force. An example of a
third-class lever is a person's arm. The fulcrum is the elbow, the biceps
provide the resistance and the effort is whatever is being lifted. The
distance from the elbow to the biceps is the effort arm and the distance from
the elbow to whatever one is trying to lift is the resistance arm. A
third-class lever is always a disadvantage.
The mechanical advantage for a lever, like all other Simple machines is the resistance force divided by the effort force (Fe/ Fr). Levers change the force that is applied to them in a similar way but not to the same extent. The ratio of the effort arm's length to the resistance arm's length tells us how much the effort force is multiplied (or how much mechanical advantage we get). The IMA for a lever is
Le is the length of the effort arm whereas Lr is the length of the resistance arm.
Levers can increase our effort forces but there must be a trade-off to equalize the situation. For example, with a crowbar, a person's strength is multiplied to pry open whatever the resistance is. However, when a person is finished he or she has pushed the crowbar a long way but the box is only opened a little bit. The crowbar takes a person's work and changes it into its own work with a large force and a small distance.
This Page was last edited on Thursday, July 27, 2000 .