A lever is a rigid bar that pivots around a fixed point, the fulcrum. It uses an applied input force, or effort, to move an output force, called the load. Levers are categorized into three classes based on the relative placement of the fulcrum, the effort, and the load. This article focuses specifically on the mechanics of the third class.
Understanding the Three Classes of Levers
The position of the middle component determines the lever class. In a first-class lever, the fulcrum is situated between the effort and the load, like a seesaw. Second-class levers have the load positioned in the middle, between the fulcrum and the effort, such as a wheelbarrow. This arrangement always results in the effort arm being longer than the load arm, which multiplies the force applied. The third-class lever is distinct because the effort is the component located in the middle.
Defining the Third Class Lever
The defining characteristic of a third-class lever is that the effort is applied at a point between the fulcrum and the load. This specific arrangement is often remembered by the sequence F-E-L. Because the effort is always closer to the fulcrum than the load, the effort arm is inevitably shorter than the resistance arm. This structural geometry dictates how the lever functions, prioritizing speed and distance over force multiplication.
Common Examples in Everyday Life
The Human Forearm
The human forearm provides an anatomical example of a third-class lever when lifting an object. The elbow joint acts as the fulcrum. The effort is supplied by the biceps muscle, which inserts on the forearm bone (radius) a short distance from the elbow. The load is the weight held in the hand, located at the far end of the forearm.
Tweezers and Tongs
A common tool operating on this principle is a pair of tweezers or tongs. The hinged end where the two arms meet serves as the fulcrum. The effort is applied by the fingers, which squeeze the arms together near the fulcrum. The load is the small object gripped at the far tips of the arms.
Fishing Rod
A fishing rod is a mechanical example where the angler’s hand holding the base of the rod acts as the fulcrum. The effort comes from the other hand pulling the rod upward, applied somewhere in the middle of the shaft. The load is the tension from the fishing line or the fish itself, located at the far end of the rod.
Mechanical Advantage and Function
The physical arrangement of the third-class lever, where the effort arm is shorter than the load arm, means it always has a mechanical advantage of less than one. A mechanical advantage less than one indicates that the input force, or effort, must be greater than the output force, or load, in order to achieve movement. This configuration trades away force amplification for a different functional benefit.
Third-class levers are designed to increase the speed and distance of the load’s movement. A small movement of the effort near the fulcrum results in the load moving a greater distance and at a higher speed. For instance, a small contraction of the biceps muscle results in the hand moving through a wide arc at a rapid pace. This speed and range of motion are why this type of lever is prevalent in biological systems and tools designed for agility or reach, rather than for lifting heavy weights.