I looked at this on intergoogle and while there were some example  
diagrams, I did not get a sense of what a rule of thumb for  
determining the mechanical advantage of pulleys.

Assume the rope and pulleys have zero mass and no friction.
Assume that I weigh 100 grams.

I attach myself to the floor and pull on a rope that goes over a  
pulley ( attached to the ceiling ) and has has an object attached to  
it.  The object is gravitationally attracted to the ground with a  
force of one newton.
            I have to pull with a force of 1 newton to lift the  
weight .... correct ?

Now I detach myself from the floor and move the rope to a hook at the  
top of my head.  Now I have to pull with a force of .5 newtons to lift  
myself.  .... correct ?


Next I attach a rope to my head , run it up through side A of a double  
pulley ( attached to the ceiling ) , back down to a pulley attached to  
my head , back up through side B of the double pulley and back down to  
my hands.   I have to pull with a force of .25 newtons to raise  
myself ........  correct ?

So what is the rule of thumb ?

Gus
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