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Chapter 2 Block and Tackle | Up
Basic Machines - Intro to machines and motion theories | Next
Figure 2-6.-A gun tackle. |
Figure 2-3.-No advantage.
MECHANICAL ADVANTAGE
With a single fixed sheave, the force of your down
pull on the fall must be equal to the weight of the object
hoist. You can’t use this rig to lift a heavy load or
resistance with a small effort-you can change only the
direction of your pull.
A single fixed block is a first-class lever with equal
arms. The arms (EF and FR) in figure 2-3 are equal;
hence, the mechanical advantage is 1. When you pull
down at A with a force of 1 pound, you raise a load of 1
pound at B. A single fixed block does not magnify force
nor speed.
You can, however, use a single block and fall to
magnify the force you exert. Notice in figure 2-4 that
the block is not fixed. The fall is doubled as it supports
the 200-pound cask. When rigged this way, you call the
single block and fall a runner. Each half of the fall carries
one-half of the total bad, or 100 pounds. Thus, with the
runner, the sailor is lifting a 200-pound cask with a
100-pound pull. The mechanical advantage is 2. Check
this by the formula:
Figure 2-4.-A runner.
2-2
Figure 2-5.-It’s 2 to 1.
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