Question

A block and tackle system has a velocity ratio of $4$. If a man can exert a pull of $240\text{kgf}$, what is the maximum load he can raise with this pulley system if its efficiency is $80\%$? Also, determine the number of pulleys used in the system.

• A. $768\text{kgf};4$
• B. $480\text{kgf};5$
• C. $112\text{kgf};6$
• D. $960\text{kgf};4$

Answer 1

The correct option is A $768\text{kgf};4$

Given:
Velocity ratio, $\text{VR}=4$
Effort, $E=240\text{kgf}$
Efficiency, $\eta =80\%$
We know that, $\eta =\frac{\text{MA}}{\text{VR}}\times 100$
On substitution, $0.8=\frac{\text{MA}}{4}$ $⟹\text{MA}=4\times 0.8=3.2$
And, $\text{MA}=3.2=\frac{\text{Load}}{\text{Effort}}=\frac{L}{240}$
$⟹L=240\times 3.2=768\text{kgf}$
For a block and tackle system, $\text{VR}=n=4$
So, number of pulleys in the system are $4$.