The positions of the effort, fulcrum, and load determine the lengths of the lever’s effort arm and load arm. Mechanical advantage increases when the effort acts farther from the fulcrum or the load acts closer to it.
The Reasoning
A lever is a rigid structure that rotates around a fixed point called the fulcrum. In human movement, a joint commonly acts as the fulcrum, muscular force provides the effort, and the body segment or external resistance represents the load.
The turning effect of a force is its moment or torque:
For an ideal lever in equilibrium:
Therefore, mechanical advantage is:
Here, is the effort arm and is the load arm. If , then , so a smaller effort can move a larger load. If , then , requiring greater effort but producing increased movement speed and range at the load.
| Lever class | Relative positions and mechanical advantage |
|---|---|
| First class | Fulcrum lies between effort and load. Mechanical advantage may be less than, equal to, or greater than , depending on arm lengths. |
| Second class | Load lies between fulcrum and effort. The effort arm is normally longer, so . |
| Third class | Effort lies between fulcrum and load. The effort arm is shorter, so . This is common in human movement. |
A common misconception is that lever class alone gives the exact mechanical advantage. Lever class indicates the arrangement, but the actual value depends on the perpendicular effort-arm and load-arm distances.
Exam Technique
For IB SEHS subtopic B.1.4, identify all three components, classify the lever, and compare the two moment arms. When explaining performance, link low mechanical advantage to greater speed and range of motion, not simply to an “inefficient” lever.