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3o8 MECHANICS.
get a useful effect of 0.9 of the force from one part of the rope
to the next ; therefore, when friction is considered, the useful
effect in the following cases will be :
In single sheave blocks having the hauling part from the
movable block (pulling with the load as in B, Fig. 14).
IV=F(1 +0.9 + 0.92
)
W=FX 2.71
In single sheave blocks having the hauling part from the
fixed block (pulling against the load as in A, Fig. 14),
W = F (0.9 + 0.92
)
JV = FX 1.71
In double sheave blocks having the hauling part from the
movable block,
W— F (1 + 0.9 + 0.92
+ 0.93
+ 0.94
)
IV=F X 4.1
In double sheave blocks having the hauling part from the
fixed block,
JV=F (0.9 + 0.92
+ 0.93 4- 0.94
)
W — F X 3.1
Differential Pulley Blocks.
In a differential pulley block (see Fig. 15), the
proportion between the force and the weight,
when friction is neglected, is expressed by the
formula :
F = W X(R — r)
2 X R
The actual force required to lift a weight by
such a pulley block is about three times the
theoretical force, as calculated above.
Inclined Plane.
When a weight is pulled upward
on an inclined plane, as shown in Fig.
16> and the force F\% acting parallel
to the plane, the required force for
moving the body will be F = W X
sin. a plus friction, and the perpen-
dicular pressure P, against the plane
will be IV X cos. a.
—Cos.-a-
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