In nautical contexts, the assembly of wheel, axle, and supporting shell is referred to as a "block."
A pulley is a wheel on an axle or shaft, a block include two or more sheaves.
The fixed pulley only changes the direction of the force, it cannot help reduce effort.The movable pulley can not change the direction of force, but can save half of the force to pull the object. The pulley group combines the fixed pulley and the movable pulley, so that both the direction of the force can be changed and the object can be lifted very effortably. Excluding the extra work done in the use of the pulley block, the more effort the moving pulley will be.
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