If a person pushes down the resistance goes up and, likewise, if a person pushes up, the resistance goes down.  An example of a first-class lever is a seesaw.  The person applying the effort is the one pushing down and the resistance is the person that the other person is trying to lift up.  The fulcrum is the point on which the seesaw goes up and down.  Depending on where the force is located, a first-class lever can be an advantage and a disadvantage.   

    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.

    In 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

  L_e 
L_r

    L_e is the length of the effort arm whereas L_r 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.