Pulley System: Calculate Work, Force, Efficiency

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The discussion revolves around calculating work, force, and efficiency in a pulley system lifting a 100N load. The total work done by the system is calculated as 1000 N, while the total force required is determined to be 120N, considering the weights of the pulleys. Participants emphasize the importance of identifying and labeling all forces in the diagram to solve the problem effectively. The conversation also highlights the relationships between the tensions in the ropes of the pulleys, leading to a better understanding of the system's mechanics. Overall, the discussion aids in clarifying the calculations and concepts involved in analyzing pulley systems.
thereddevils
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Homework Statement



The diagram shows a pulley system consisting of two pulleys of weights 10 N each. The system is doing work to lift a load of 100N upwards for 10m at constant speed.
Calculate (1) the total work done by the pulley system. (2) the magnitude of force,F
(3) the efficiency of the system.

Homework Equations





The Attempt at a Solution



(1) W= Fs = 1000 N

(2) F=100+10+10=120N

(3) not sure.
 

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hi thereddevils! :smile:
thereddevils said:
(2) F=100+10+10=120N

not even close :redface:

in questions like this, always mark all the forces on the diagram first :wink: …

in this case, there are four tensions (one of which is F) …

what are they? :smile:​
 
tiny-tim said:
hi thereddevils! :smile:


not even close :redface:

in questions like this, always mark all the forces on the diagram first :wink: …

in this case, there are four tensions (one of which is F) …

what are they? :smile:​

Thanks Tim,

Do i solve this by breaking it into layers? The tensions in the string of the bottom pulley equals the weight of the bottom pulley itself plus the load. Then, the tension in the string of the upper pulley equals the its own weight + the weight of the bottom pulley and the load. This is what i had in mind.

Yes,i see the four tensions but what do they equal to?
 
Give the tensions names (T1 T2 T3 and F), and write the equations for each pulley, starting with the lower one …

what do you get? :smile:​
 
tiny-tim said:
Give the tensions names (T1 T2 T3 and F), and write the equations for each pulley, starting with the lower one …

what do you get? :smile:​

ok let me try

Let T1 and T2 be the tensions of the strings of the bottom pulley, and T3 be the tension of the string of the upper pulley.

T1 + T2 = 110 (load+weight of pulley)

T3 + F = T2 + 10

Am i somewhere near?
 
Perfect! :smile:

ok, now (i'm assuming the pulleys are "frictionless") the tension in the rope on either side of a pulley is the same,

so T1 = T2, and T3 = F, and so … ? :wink:
 
tiny-tim said:
Perfect! :smile:

ok, now (i'm assuming the pulleys are "frictionless") the tension in the rope on either side of a pulley is the same,

so T1 = T2, and T3 = F, and so … ? :wink:

Thanks Tiny Tim!
 

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