Simplest Atwood machine question.

In summary, the question asks how hard a person should pull a string in order to raise a bucket with constant speed. The system must have 0 acceleration, leading to the conclusion that the person should exert half their weight in force. However, in real life, friction must also be taken into account.
  • #1
life is maths
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Homework Statement


Hi, the question is like in the picture. How hard should she pull the string to raise with constant speed? She weighs 70 kg together with the bucket.


The Attempt at a Solution


The system must have 0 acceleration, but does this mean she should pull the string with an acceleration equal to g, to balance with it? Or is it enough that she pulls equal to her weight? But in this case, she would not raise.
This is a simple question, I guess, but I'm confused. Thanks for any help :)
 

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  • #2
I had this same question a few days ago. You just have to realize that the force she puts on the rope will lift her through the pulley, but also lift her directly. This lead me to believe that she would only need exert half her weight in force. I tried F=(1/2)mg, and it was correct.
 
  • #3
The person exerts some force F, and this force evolves tension T in the rope: T=F. The tension is the same all along the rope, so the resultant force from the rope on the system (bucket+person) is equal to 2T. If 2T = mg, (weight of the person), that is F=mg/2, the bucket is balanced, but just a bit larger force is enough to move the person upward.

In real life, there is friction, so the person needs to exert some more force to overcome friction.

ehild
 
  • #4
Thanks a lot, ndifabio and ehild, I cannot believe that it was that simple. Your help is greatly appreciated :)
 
  • #5
Any time, I'm always happy to help a fellow Physics student :)
 

1. What is an Atwood machine?

An Atwood machine is a simple mechanical device that consists of two masses connected by a string or pulley. It is used to demonstrate the principles of Newton's laws of motion and to calculate the acceleration of the system.

2. How does the Atwood machine work?

The Atwood machine works by balancing the forces acting on the two masses connected by a string or pulley. The force of gravity on each mass causes the system to accelerate in a specific direction until the forces are balanced, resulting in a constant velocity.

3. What is the simplest Atwood machine question?

The simplest Atwood machine question typically involves calculating the acceleration of the system or determining the tension in the string or pulley. It may also involve solving for the unknown mass or distance if given the other variables.

4. What are the factors that affect the acceleration in an Atwood machine?

The acceleration in an Atwood machine is affected by the difference in mass between the two objects, the force of gravity, and the friction or resistance in the system. The angle of the pulley or string can also affect the acceleration.

5. How is the Atwood machine used in real life?

The Atwood machine is used in various applications, such as elevators, cranes, and hoists, to lift or move heavy objects. It is also used in physics experiments and demonstrations to teach concepts of motion and forces.

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