Question

How many movable pulleys are there in a system of 3 pulleys? What is the mechanical advantage and velocity ratio of this system? If the efficiency of the system is 100%, what effort is needed to lift a load of 350 kgf attached to it?

Answer 1

Given:

Total number of pulleys (n) = 3

Efficiency (η) = 100% = 1 (ideal system)

Load (L) = 350 kgf

1. Number of Movable Pulleys:

In a standard block and tackle system with an odd number of total pulleys (3), the pulleys are distributed such that the upper fixed block contains one more pulley than the lower movable block.

Fixed (Top) block = 2 pulleys

Movable (Bottom) block = 1 movable pulley

There is 1 movable pulley in the system.

2. Velocity Ratio (VR) and Mechanical Advantage (MA):

For a block and tackle system, the velocity ratio is always equal to the total number of pulleys in the system.

VR = 3

Since the efficiency is perfect (100%), the mechanical advantage is equal to the velocity ratio.

MA = Efficiency × VR

= 1 × 3

= 3

3. Calculation of Effort (E):

Using the formula for mechanical advantage:

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

3 = `350/E`

E = `350/3`

E = 116.67 kgf

E = 117 kgf