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MECHANICS. 307
150 lbs-
Pulley Blocks.
When friction is not considered, the fig. 13.
force and the load will be equal in a single
fixed pulley (as A, Fig. 13).
Thus, a single fixed pulley does not
accomplish anything further than to change
the direction of motion. In a single movable
pulley (as fat B, Fig. 13), the force is equal
to only half the load; thus, 75 pounds of
force will lift 150 pounds of load, but the
force must act through twice the space that
the load is moved. The tension in any part
of the rope in B is half of the load W\ thus, when the load is
150 pounds the tension in the rope is 75 pounds, when arranged
at B, but it is 150 pounds when arranged as at A.
Fig. 14 shows a pair of single sheave pulley blocks in
position to pull a car ; when the blocks are arranged as at Ay
and friction is not consid-
ered, a force of 100 pounds
on the hauling part of the
rope exerts a force of 300
pounds on the post, but only
200 pounds on the car ; but,
turning the blocks end for
end, as shown at B, a force
of 100 pounds on the hauling
part of the rope exerts a
force of 300 pounds on the car and 200 pounds on the post.
This is a point well worth remembering when using pulley blocks.
Suppose, for instance, that a man exerted a force of 100
pounds on the hauling part, and that it required 250 pounds
of force to move the car ; if he used the pulley blocks as
shown at A, his work would be useless, as far as moving the car
is concerned, as he could not do it, but turning his blocks end
for end he could accomplish the desired result. Always re-
member whenever it is possible to have the hauling part of the
rope coming from the movable block and pull in the same
direction as the load is moving.
Friction in Pulley Blocks.
In practical work, friction will have some influence, and, to
a certain extent, change these results, because some of the ten-
sion in the rope is lost by friction in each sheave the rope passes
over, therefore the tension in each following part of the rope is
always less than it was in -the preceding part. This loss must
be obtained from experiments. In good pulley blocks, having
roller bearings, this loss is probably not more than 0.1, and we
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