Draw a diagram of combination of three movable pulleys with a fixed pulley showing the directions of load, effort and tension in each strand. Find: (i) mechanical advantage, (ii) Velocity ration and (iii) efficiency of the combination in ideal situation.
Tension T1 in the string passing over the pulley A is given as
2T1 = L or T1 = L/2
Tension T2 in the string passing over the pulley B is given as
2T2 = T1or T2 = T1/2 = L/22
Tension T3 in the string passing over the pulley C is given as
2T3 = T2 or T3 = T2/2 = L/23
In equilibrium, T3 = E
E = L/23
Mechanical advantage = MA = L/E = `2^3`
As one end of each string passing over a movable pulley is fixed, so the free end of string moves twice the distance moved by the movable pulley.
If load L moves up by a distance x, dL = x, effort moves by a distance `2^3`x, dE = `2^3`x
Velocity Ration VR = `"Distance moved by the effort dE "/"Distance moved by the load dL "` = `(2^3 x)/x= 2^3`
Efficiency = MA/VR =`2^3`/`2^3` = 1 OR 100%