Hi encounter pulley system while teaching my tuition gal on forces and simple machine. So went to check out how to know the effort needed to lift the weight in different pulley system.
The mechanical advantage of each system is easy to determine. Count the number of rope segments on each side of the pulleys, including the free end. If the free end is to be pulled down, subtract 1 from this number. This number is the mechanical advantage of the system! To compute the amount of force necessary to hold the weight in equilibrium, divide the weight by the mechanical advantage!
Example in P=W, there is 1 string attached to pulley and 1 free end. Total no.s of rope segment will be 2 but it is being pulled down so substract 1. Therefore the effort needed is Weight / (2-1) = Weight/ 1. So force needed is the same as the weight.
In P=1/2 W, there is 1 string attached to pulley and 1 free end but is not being pulled down. So effort needed is Weight / 2, therefore you will only require half of the effort to move the weight.
In P= 1/4 W, there is 4 string attached to pulley and 1 free end, but it is being pulled down so substract 1. Therefore the effort needed is Weight / (5-1) = Weight/ 4. Therefore you will only require a quarter of the effort to move the weight.
Anyway special thanks to SWE organisation which gives such easy to understand formula to let us learn about pulley system. Those who want to test yourself on pulley system can also go to this link http://www.swe.org/iac/lp/pulley_act.html. Hope you have learn a lesson on it.
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