Question

Figure shows a system of four pulleys. The upper two pulleys are fixed and the lower two are movable.

1. Draw a string around the pulleys. Also show the place and direction in which the effort is applied.
2. What is the velocity ratio of the system?
3. What is the mechanical advantage of the system?
4. What assumption do you make in arriving at your answer in part (iii)?

Answer 1

(i) Pass one continuous string alternately around the pulleys: fix one end to the upper block, take it down around the first movable pulley, up over the first fixed pulley, down around the second movable pulley, and then up over the second fixed pulley. Apply the effort vertically downward at the free end of the string.

(ii) The velocity ratio is equal to the number of strings supporting the movable block: 

V.R. = 4

There are four supporting strands.

(iii) For an ideal pulley system,

M.A. = V.R. = 4

M.A. = 4

(iv) The pulleys are assumed to be frictionless and the string is light and inextensible. Hence, the tension is the same throughout the string and the efficiency is taken as 100 %.