Question

A ‘block and tackle’ system used 3 pulleys in the lower block and 4 pulleys in the upper block. What is the ‘velocity ratio’ of this system? If the load is to be lifted by a person capable of applying a maximum effort of 1000 N, what is the maximum load than can be lifted under ideal conditions?
The actual maximum load that gets lifted turns out to be 6300 N. What are the values of the actual M.A. and efficiency of the set-up?

Answer 1

There being 7 pulleys in all, the load is supported by 7 strands of string. Hence it gets lifted only through 1/7 th of the distance through which the effort is applied. Hence the velocity ratio of the setup has a value 7.
Under ideal conditions, M.A. = V.R. = 7
Hence, the maximum load that can be lifted under ideal conditions is:

Effort × 7 = 1000 N × 7 = 7000 N.

Under actual conditions, Load = 6300 N

and Effort = 1000 N

Hence, Actual M.A. =`(6300"N")/(1000"N")=6.3`

Also Efficiency =`"M.A."/"V.R."=6.3/7`

= 0.9 = 90%.