Bosun's chair problem - force on the seat?

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In summary, the boy is lifting himself up by pulling down on a rope, which also pulls up the chair with the same upward force.
  • #1
Stinky
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A friend and I have been having some difficulties with a problem involving a bosun's chair. A boy lifts himself up by pulling down on the rope with a force T, which lifts him up the ground. The problem is asking what would the force on the chair be as he is lifting. Our initial thought would be that it is the same as his weight. He is pulling down with T, which at the same time pulls himself up with T, and also the chair with T. Since the same upward force is added to his weight and also to the force of the chair against him, we assumed the force on the chair is equal to his weight. However, we submitted this online and it said it was wrong. Any ideas?
 
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  • #2
It might be easier to understand if you think of person + chair as a single object. At how many points does the rope pull up on that object? In terms of rope tension, what's the upward force on the object? And how does the force with which the man pulls on his end of the rope relate to the rope tension?
 
  • #3
Stinky said:
A friend and I have been having some difficulties with a problem involving a bosun's chair. A boy lifts himself up by pulling down on the rope with a force T, which lifts him up the ground. The problem is asking what would the force on the chair be as he is lifting. Our initial thought would be that it is the same as his weight. He is pulling down with T, which at the same time pulls himself up with T, and also the chair with T. Since the same upward force is added to his weight and also to the force of the chair against him, we assumed the force on the chair is equal to his weight. However, we submitted this online and it said it was wrong. Any ideas?

your question is not clear enough. Do you mean the net force on the chair? Do you mean the normal force on the chair? Are you neglecting the weight of the chair? Is the boy moving up at constant speed?
 
  • #4
Stinky said:
A friend and I have been having some difficulties with a problem involving a bosun's chair. A boy lifts himself up by pulling down on the rope with a force T, which lifts him up the ground. The problem is asking what would the force on the chair be as he is lifting. Our initial thought would be that it is the same as his weight. He is pulling down with T, which at the same time pulls himself up with T, and also the chair with T. Since the same upward force is added to his weight and also to the force of the chair against him, we assumed the force on the chair is equal to his weight. However, we submitted this online and it said it was wrong. Any ideas?

I am going to assume that you are looking for the nromal force on the chair and that the speed is constant and the weight of the chair is negligible.

A FBD of the chair will then contain two forces: the normal of the boy on the chair and the tension upward.

The FBD of the boy shows three forces: gravity down, a normal force up and a tension up (he pulls down on the rope so the rope pulls up on him).

If you now combine the two as a single object, what FBD do you get? Then the answer will be obvious.
 
  • #5
Sorry, I will be clearer. He is accelerating upward at a constant rate. I am trying to figure out the force that the boy exerts onto the chair as he and the chair are accelerating upwards.
 
  • #6
nrqed gave you a systematic approach to follow that can easily be modified to incorporate acceleration: Draw a free body diagram and apply Newton's 2nd law.

But I'd like you to first understand the key idea of the bosun's chair. Forget acceleration for a moment and assume the chair itself weighs nothing. How hard does he have to pull to support his weight? (Read what I wrote above.)
 
  • #7
He will have to pull with a force equal to half of his weight to support his weight, correct?
 
  • #8
Stinky said:
He will have to pull with a force equal to half of his weight to support his weight, correct?
Exactly. So what provides the other force (half of his weight) needed to support him?
 
  • #9
He provides half of it by pulling down on the rope, and therefore upwards on himself, and the chair pushes up with the other half.
 
  • #10
Stinky said:
He provides half of it by pulling down on the rope, and therefore upwards on himself, and the chair pushes up with the other half.
Exactly. Whatever total upward force is needed on the man, half is provided by the rope and half by the normal force of the seat. So, what upward force is needed if he is accelerating?
 
  • #11
A total force greater than his weight, or he must pull with a force greater than half his weight.
 
  • #12
Stinky said:
A total force greater than his weight, or he must pull with a force greater than half his weight.
Right. Now answer the question in terms of the information given in the problem statement (from your first post). It says: He pulls down on the rope with a force T.
 

FAQ: Bosun's chair problem - force on the seat?

1. What is a Bosun's chair and how does it work?

A Bosun's chair is a type of seat used by sailors and workers in various industries to perform tasks at heights. It consists of a wooden or metal seat attached to a rope or cable, which is then suspended from a higher point, allowing the user to be raised or lowered. The user's weight is supported by the chair and the force on the seat is calculated based on the tension in the rope or cable.

2. How is the force on the seat of a Bosun's chair calculated?

The force on the seat of a Bosun's chair is calculated using Newton's second law of motion, which states that force is equal to mass multiplied by acceleration. In this case, the acceleration is due to gravity, and the mass is the combined weight of the user and the chair. By measuring the tension in the rope or cable and accounting for the angle of the rope, the force on the seat can be accurately determined.

3. What factors affect the force on the seat of a Bosun's chair?

The force on the seat of a Bosun's chair is affected by several factors, including the weight of the user and the chair, the angle of the rope or cable, and any external forces such as wind or movement. Additionally, the type and condition of the rope or cable, as well as the strength and stability of the attachment point, can also impact the force on the seat.

4. Is there a maximum weight limit for a Bosun's chair?

Yes, there is typically a maximum weight limit for a Bosun's chair, which is determined by the strength and stability of the chair and its components. It is important to always follow the manufacturer's guidelines and weight limits to ensure the safety of the user and the equipment.

5. Are there any safety precautions to consider when using a Bosun's chair?

Yes, there are several safety precautions to consider when using a Bosun's chair. These include checking the condition and strength of the equipment before each use, following weight limits and guidelines, using proper safety harnesses and attachments, and having a spotter or helper on the ground. It is also important to use caution in adverse weather conditions and to be aware of any potential hazards or obstacles in the work area.

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