Question

In a block and tackle system consisting of 3 pulleys, a load of 75 kgf is raised with an effort of 25 kgf, find the efficiency.

Answer 1

Given:
The block and tackle system consists of 3 pulleys, i.e.$n=3$
Effort $E=25kgf=250N$
Load $L=75kgf=750N$
The mechanical advantage is equal to the number of pulleys in both the blocks.
$M.A.=\frac{loadL}{effortE}=\frac{750N}{250N}=3$
In a system of 𝑛 = 3 pulleys, if the load moves up through a distance d, each segment of string supporting the load gets loosened by a length d.
So if $d_L=d$ then distance moved by the effort
end is $d_E=nd=3d$.
So the velocity ratio is
$V.R.=\frac{d_E}{d_L}=\frac{nd}{d}=n=3$
𝐸𝑓𝑓𝑖𝑐𝑖𝑒𝑛𝑐𝑦, $\eta =\frac{M.A.}{V.R.}=\frac{3}{3}=1\text{or}100$ %
So the efficiency of the block and tackle
system is 100 %.