Figure 2-3.-No advantage.MECHANICAL ADVANTAGEWith a single fixed sheave, the force of your downpull on the fall must be equal to the weight of the objecthoist. You can’t use this rig to lift a heavy load orresistance with a small effort-you can change only thedirection of your pull.A single fixed block is a first-class lever with equalarms. The arms (EF and FR) in figure 2-3 are equal;hence, the mechanical advantage is 1. When you pulldown at A with a force of 1 pound, you raise a load of 1pound at B. A single fixed block does not magnify forcenor speed.You can, however, use a single block and fall tomagnify the force you exert. Notice in figure 2-4 thatthe block is not fixed. The fall is doubled as it supportsthe 200-pound cask. When rigged this way, you call thesingle block and fall a runner. Each half of the fall carriesone-half of the total bad, or 100 pounds. Thus, with therunner, the sailor is lifting a 200-pound cask with a100-pound pull. The mechanical advantage is 2. Checkthis by the formula:Figure 2-4.-A runner.2-2Figure 2-5.-It’s 2 to 1.
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