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Pulleys reduce the work needed 
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#1
Apr2604, 08:18 PM

P: 204

clarify this for me...
I know that pulleys reduce the work needed, depending on how its setup... But lets say at you add n number of pulleys to mass... And the work is reduced by 1/n... So you add infinite amount of pulleys... and then you supply an infinity amount of force... inf/inf = ? What exactly are you getting here? Tug 5m... with 5 pulleys in this setup its just 5/5...hmmm confused. 


#2
Apr2604, 09:56 PM

P: 991

Just a little warning: Stay away from infinities. They do nothing productive.
cookiemonster 


#3
Apr2604, 10:12 PM

P: 204

Gravity... couldn't you use that to supply the pull of this pulley?



#4
Apr2604, 10:18 PM

P: 991

Pulleys reduce the work needed
Er... Gravity...?
The problem is an infinite amount of force is a tricky thing to handle. Combine it with an infinite number of pulleys and we have an even more difficult (i.e. impossible) problem to handle. Simply put, infinity doesn't exist in physics, so don't try to force it in there. cookiemonster 


#5
Apr2604, 10:25 PM

P: 204

Whats about finite? what if the rope itself can expand or shrink... and you have to tug at it as it shrinks... then wouldn't you do less work? or... am I confusing myself...



#6
Apr2604, 10:26 PM

P: 991

I don't know about yourself, but you're certainly confusing me. =\
cookiemonster 


#7
Apr2604, 10:46 PM

P: 204

heh... nvm then.



#8
Apr2704, 08:48 AM

Sci Advisor
P: 789

Work would be the SAME except for increased losses.
Force decreases with the addition of pulleys and rope displacement increases. You would quickly exceed the rope's strength well before reaching infinity, and at some point the drag of all the pulleys and sheer mass of rope between them needs to be included in the calculation. Just having an infinite length of rope would have huge mass and then an infinite force would cause the rope to exceed its tensile strength and break, no pulley's needed. Cliff 


#9
Apr2704, 09:38 AM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,353

If you attached an "infinite" number of pulleys you would use 0 force for an "infinite" distance which means nothing, physically. As CookieMonster said, "Stay away from infinities"! 


#10
Apr2704, 11:39 AM

Mentor
P: 22,248

[beaten to death, but eh...] 


#11
Apr2704, 07:13 PM

P: 204

alright... thanks for the clarification.



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