The calculation method is the same for all three lever classes: balance the moments around the fulcrum. The difference is the relative position of the fulcrum, effort, and load, which determines the lever's mechanical advantage.
A moment is the turning effect of a force about a pivot:
For a lever in equilibrium, the clockwise and anticlockwise moments are equal:
Here, is the effort force, is the effort arm, is the load force, and is the load arm. Distances must be measured perpendicularly from the fulcrum to each force's line of action.
| Lever class | Arrangement and calculation consequence |
|---|---|
| First class | Fulcrum between effort and load. Either arm may be longer, so the ideal mechanical advantage may be less than, equal to, or greater than . |
| Second class | Load between fulcrum and effort. The effort arm is longer, so ideal mechanical advantage is greater than . |
| Third class | Effort between fulcrum and load. The effort arm is shorter, so ideal mechanical advantage is less than , but movement and speed are increased. |
For example, lifting a load with a load arm of and an effort arm of requires:
This geometry could represent a first- or second-class lever, depending on component order. In a third-class lever with and , the same load requires .
Ideal mechanical advantage can also be found from arm lengths:
This explains the patterns. In the first example, , so the effort is half the load. In the third-class example, , so effort is four times the load. Actual mechanical advantage may be lower because friction and deformation cause energy losses.
Exam technique: Draw and label the fulcrum, effort, and load before substituting values. A common misconception is that each class uses a different equation; the equation is unchanged, while the component arrangement and arm lengths differ.
