Second-class lever

Two other types of levers exist, hi one, called a second-class lever, the resistance force lies between the effort force and the fulcrum. A nutcracker is an example of a second-class lever. The fulcrum in the nutcracker is at one end, where the two metal rods of the device are hinged together. The effort force is applied at the opposite ends of the rods, and the resistance force, the nut to be cracked open, lies in the middle. 

Figure 13 (a) Example of the first-class lever system, (b) Example of the second-class lever system, (c) Example of the third-class lever system and single-segment static analysis. (Adapted from Troup and Edwards, reproduced with permission of the Controller of Her Majesty s Stationery Office)... 

Machine screws (or bolts) require special tools. The combination wrench is one type of hand tool used with machine screws. The adjustable wrench is more versatile, allowing the technician to use it with different sizes of machine screws (see Figure 8-20). We may also use pliers, second-class levers, during the combining process. Pliers can be found in various types (see Figure 8-21). Slip joint pliers allow material to be held firmly. Needle nose pliers can hold smaller items. Diagonal pliers (cutters) are actually a shearing tool for wire. 

A second-class lever would he the wheelbarrow where the wheel serves as the fulcrum (Figure 10-7). The length of the lever arms and the position of the load determine the mechanical advantage gained. A third-class lever is... 

FIGURE 10-7 The second-class lever has its fulcrum at the opposite end of the arm from where the force is applied. A wheelbarrow is an example of a second-class lever. 

A wheel and axle is a rotating lever (the wheel) that moves around the fulcrum (the axle). The wheel and axle utilizes the mechanical advantage of a second-class lever. The larger the wheel diameter, the greater the mechanical advantage that is gained (Figure 10-9). Examples of this second-class lever (wheel and axle) include a door knob and the pedal spindle on your bicycle. 

A second-class lever has the resistance placed between the fulcrum and the point of effort. 

Of the three levers, the second-class lever is most efficient with regard to minimizing effort needed to move a resistance. A wheelbarrow is an example of such a lever. Most of our long muscles act as third-class levers. The biceps muscle s flexion effort in moving the forearm is... 

In a third-class lever, the effort force lies between the resistance force and the fulerum. Some kinds of garden tools are examples of third-class levers. When you use a shovel, for example, you hold one end steady to act as the fulcrum, and you use your other hand to pull up on a load of dirt. The second hand is the effort force, and the dirt being picked up is the resistance force. The effort applied by your second hand lies between the resistance force (the dirt) and the fulcrum (your first hand). 

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