Third-class levers
If you look carefully at the line drawing of a third-class lever in Figure 10, you will see that the effort is applied between the load and the fulcrum. This means that if the distance from the effort to the fulcrum is smaller than the distance from the load to the fulcrum, we will need to exert a greater effort to move a smaller load. The mechanical advantage is that the effort is over a short distance but the load moves over a greater distance.
Figure 10 Line diagram of a third-class lever
Figure 11 This principle was used in this design for an old-fashioned catapult used as a fighting machine. A rope applies the effort to pull the beam down and this creates tension in a tightly wound rope at the bottom of the beam. When the first rope is released, the rock is thrown towards the target.
Figure 12 A modern equivalent of this type of action is the fishing rod. The fisherman holds the bottom of the rod. This is the fulcrum point. He pulls the rod backwards – this is the effort. He then flings the rod causing the line with the hook on it to travel a long way out into the sea or river.
Figure 13 A pair of tweezers is an example of combined third-class levers. A small movement of the fingers on the middle produces a larger movement on the tip in order to grip a hair firmly. The load is the resistance of the hair to being pulled out.
104 Topic 5 Mechanical systems and control
Figure 14 Sometimes levers work together in a machine. The mechanism that connects the keys on a piano keyboard to cause the strings of the piano to be struck is a combination of levers.
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