Confused about how forces in a pulley balance

In summary, the balance of forces in a pulley system involves understanding how tension and weight interact. When a pulley is used to lift an object, the tension in the rope counteracts the gravitational force acting on the object. If the pulley is frictionless and the system is in equilibrium, the forces are balanced, meaning the tension equals the weight of the object. For systems with multiple pulleys or masses, the analysis requires applying Newton's laws to determine how forces distribute among the components, ensuring that the total upward force matches the total downward force for balance.
  • #1
sdfsfasdfasf
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Homework Statement
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Relevant Equations
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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
 
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  • #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.
 

FAQ: Confused about how forces in a pulley balance

1. How do forces in a pulley system balance?

In a pulley system, forces balance when the tension in the rope and the weight of the objects being lifted are equal. For example, if you have a mass hanging from a pulley, the gravitational force acting on the mass (weight) is countered by the tension in the rope. If the system is in equilibrium, these forces will be equal, resulting in no net acceleration.

2. What happens when one side of the pulley has more weight?

When one side of the pulley has more weight, the system will not be balanced. The heavier side will accelerate downward while the lighter side will move upward. The difference in weight creates an unbalanced force, resulting in motion until the forces are balanced again or until the system reaches a new equilibrium position.

3. Can pulleys change the direction of force?

Yes, pulleys can change the direction of the applied force. For instance, if you pull down on one side of a pulley, the other side will move up. This is useful in lifting objects, as it allows you to exert force in a more convenient direction without changing the amount of force needed to lift the object.

4. How does friction affect the forces in a pulley system?

Friction can significantly affect the forces in a pulley system by opposing the motion of the rope and the pulley. This means that more force may be required to lift an object than just the weight of the object alone. The presence of friction can lead to a decrease in the efficiency of the system and may require additional input force to maintain balance.

5. What is the role of a fixed vs. movable pulley in balancing forces?

A fixed pulley changes the direction of the force but does not change the amount of force needed to lift an object. A movable pulley, on the other hand, can reduce the amount of input force required to lift a load by distributing the weight across multiple segments of rope. In both cases, understanding how the forces balance is crucial for determining the required force to lift an object effectively.

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