Question

A system of pulleys consists of three fixed and two movable pulleys.

1. What is the mechanical advantage of this system?
2. How much effort is required to pull 20 kgf through this system?
3. If the efficiency of the system is 70%, what is its velocity ratio?

Answer 1

Given:

Number of fixed pulleys = 3

Number of movable pulleys = 2

Total number of pulleys (n) = 3 + 2 = 5

Load (L) = 20 kgf

(i) For an ideal block and tackle system, the Mechanical Advantage is equal to the total number of pulleys in the system.

MA = 5

(ii) MA = `"Load (L)"/"Effort (E)"`

`5 = 20/E`

E = `20/5`

E = 4 kgf

(iii) Efficiency (η) = 70% = `70/100` = 0.70

MR = Efficiency (η) × Velocity Ratio (VR)

VR = `(MA)/η`

= `5/0.70`

= 7.14