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I've just started learning about pulleys in physics and wondering if below setup offers mechanical advantage. The forces balance nicely but it looks too simple to work. If I pull the rope with a force of $\dfrac{Mg}{4}$, would the masses move up?
(Assume the rope and the two horizontal bars are massless and frictionless)

enter image description here

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What you have described is a double tackle assembly, which gives a W/4 (4 to 1) lift advantage. Frictionless, ideal rope and bars would work the same as ideal pulleys, assuming the ropes do not slip off of the bars. see https://en.wikipedia.org/wiki/Block_and_tackle

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  • $\begingroup$ Wow so it works xD Thank you so much :) Looks magic how the input $\dfrac{Mg}{4}$ force copies itself $4$ times at the load! $\endgroup$ – AgentS Dec 8 '19 at 11:02
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    $\begingroup$ For example: In your ideal system pulling 4 feet of rope with 1 newton force would raise a 4 newton weight 1 foot. (discounting initial acceleration) $\endgroup$ – Adrian Howard Dec 8 '19 at 11:13
  • $\begingroup$ It works, but it is NOT a question of pulleys, here. As Adrain Howard says, it has to do with tackles, NOT pulleys $\endgroup$ – Alfred Dec 8 '19 at 22:32
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My friend, It's not about pulleys, it's about the force needed, And the torque is in function of the diameter of the pulley, so you meed to have different pulleys, with different diameters in order to decrease the force needed,

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  • $\begingroup$ Ohk thank you:) Then if the rope slides with out rotating the pulley, will my setup above work? $\endgroup$ – AgentS Dec 8 '19 at 10:06
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    $\begingroup$ For an ideal pulley system (frictionless pulleys and ropes) pulley diameter Is not part of the mechanical advantage calculation $\endgroup$ – Bob D Dec 8 '19 at 10:40
  • $\begingroup$ I'm not sure why you posted two answers saying something so similar, especially since torque had nothing to do with the question. $\endgroup$ – JMac Dec 8 '19 at 20:51
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No, because in your setup, The diameter is fix, it's not increasing,

Torque = radius$×$force, The force is always the same, since the mass is fix $F=mg$

But the torque decreases when $r$ increases, That's why we use pulleys.

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    $\begingroup$ The lifting force of rope pulleys does not depend on the pully diameter. $\endgroup$ – Adrian Howard Dec 8 '19 at 11:00
  • $\begingroup$ Pulleys are objects that turn. Nothing turns here, the rope slips on the bars. If one neglects friction the rope will indeed pull the weights with a ratio 1/4 of forces. Don't think in terms of torque, since nothing turns ! $\endgroup$ – Alfred Dec 8 '19 at 22:34

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