# Pulleys reduce the work needed

1. Apr 26, 2004

### Ebolamonk3y

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. Apr 26, 2004

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

3. Apr 26, 2004

### Ebolamonk3y

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

4. Apr 26, 2004

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.

5. Apr 26, 2004

### Ebolamonk3y

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. Apr 26, 2004

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

7. Apr 26, 2004

### Ebolamonk3y

heh... nvm then.

8. Apr 27, 2004

### Cliff_J

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. Apr 27, 2004

### HallsofIvy

Staff Emeritus
Pulleys do NOT reduce "work needed"- not in the physics sense of the word "work". They reduce the force needed in "work= force times distance" by increasing the distance over which the force is applied.

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. Apr 27, 2004

### Staff: Mentor

Wise words.

[beaten to death, but eh...]
No, actually, pulleys keep the work exactly the same (assuming no losses of course). Double the number of strands and you get twice the force at half the displacement on one side and twice the displacement for half the force on the other.

11. Apr 27, 2004

### Ebolamonk3y

alright... thanks for the clarification.