Confused about how forces in a pulley balance

  • #1
sdfsfasdfasf
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12
Homework Statement
x
Relevant Equations
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View attachment 344841
The question is regarding this image, I understand the two 50N forces provide a clockwise moment of 40 x 10^-2 x 100 which gives option D, as this moment must be balanced by the moment given by T.
Now my question is, how is the net force on this system equal to 0? It says it is in equilibrium which to me means that the net force = 0 and the net moment about any point = 0 but clearly the two vertical forces cancel out (the 50N ones), and then T is left, we know T is non zero so how is the net force 0?

Thank you
 
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  • #2
Presumably the pulley is supported through its axle. This provides an additional external force which will be whatever necessary without affecting total torque.

Edit: typo fixed
 
Last edited:
  • #3
The net force would be zero if the pulley has a fixed pivot at its center. Perhaps the author of the problem thought that this is obvious.
 
  • #4
If there is a force counteracting the force T, does that not provide an opposite moment to the force T? Then why is the moment provided by T not half of the calculated value of 40?
 
  • #5
Silly me, if the force is at the center then the distance is 0 and it provided no moment.
 
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  • #6
I was about to point that out. Glad you found it on your own.
 
  • #7
sdfsfasdfasf said:
Then why is the moment provided by T not half of the calculated value of 40?
Why do you believe that it should be less than 40 N-m?
The pulley and the force T are creating a concentrated moment applied to the shaft, which should be counter-acting the other two moments acting on the same pivoting point, if the bar is not rotating.
 

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