Pulley System: Calculate Work, Force, Efficiency

[thereddevils]

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.

 

[tiny-tim]

hi thereddevils!

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

not even close

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

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

what are they? ​

 

[thereddevils]

hi thereddevils!


not even close

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

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

what are they? ​

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?

 

[tiny-tim]

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? ​

 

[thereddevils]

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? ​

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?

 

[tiny-tim]

Perfect!

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 … ?

 

[thereddevils]

Perfect!

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 … ?

Thanks Tiny Tim!