In block & tackle system the number of pulleys in the movable lower block is either equal to or one less than the number of pulleys in the fixed upper block. Why?
where w is the total weight of lower block alongwith pulleys in it. E is the effort.
Efficiency is reduced due to the weight of lower block pulleys. More the weight of lower block pulleys, less the efficiency.
Hence for greater efficiency, the pulleys in the lower block should be as light as possible.
Answers
Dear Student
Velocity ratio 3 means ratio of distance moved by effort to distance moved by load is 3.
Below the diagram of block and tackle system of pulleys having velocity ratio 3.
The lower block of this pulley system must have a negligible weight to increase the efficiency of the system.
Regards
Explanation:
In the balance position of block n tackle system
load =nT(n= number of pulleys)
Effort=T(T=tension)
Also L + w =nT(w=weight of lower pulleys)
since T =Effort
so we can write
L+ w=nE(this means that w and L are balanced by all the Tension in string. and also Load+ W.of lower pulley is the all total load of a machine )
L=nE-W
M.A=load/effort
=nE-W/E
=n-W/E
in block n tackle system, V.R=n (n=no.of pulleys)
Hence, effienciency=MA/VR
=n-W/E/n
nE-W/E ×1/n
=nE-We/n
=1-W/nE
So, from here we can say that on increasing no. of pulleys we have less effieciency as increase in no. increases the weight of lower pulleys. So, in order to have good efficiency of the machine we should always keep the no.of lower pulley always less or equal to upper pulleys.
Hope u understand!!!!