[Illustration: FIG. 110.--A movable pulley lightens labor.]
162. Combination of Pulleys. A combination of pulleys called block
and tackle is used where very heavy loads are to be moved. In Figure
111 the upper block of pulleys is fixed, the lower block is movable,
and one continuous rope passes around the various pulleys. The load is
supported by 6 strands, and each strand bears one sixth of the load.
If the hand pulls with a force of 1 pound at _P_, it can raise a load
of 6 pounds at _W_, but the hand will have to pull downward 6 feet at
_P_ in order to raise the load at _W_ 1 foot. If 8 pulleys were used,
a force equivalent to one eighth of the load would suffice to move
_W_, but this force would have to be exerted over a distance 8 times
as great as that through which _W_ was raised.
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