Basic forces question with a pulley problem

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Homework Help Overview

The discussion revolves around a problem involving forces and tension in a pulley system, specifically focusing on the tension at point A when a boy holds the rope. The problem is situated within the context of basic mechanics.

Discussion Character

  • Exploratory, Assumption checking

Approaches and Questions Raised

  • Participants explore the implications of where the boy is holding the rope, questioning how this affects the tension in the system. There is an attempt to reconcile the book's answer with personal reasoning about the forces involved.

Discussion Status

The discussion is active, with participants providing insights into how the position of the boy affects the tension in the rope. Some guidance has been offered regarding the need to consider the applied force and its impact on the tensions on either side of the pulley.

Contextual Notes

There is uncertainty regarding the exact position of the boy holding the rope, which is crucial for determining the tension accurately. Participants are also grappling with the implications of different tension values on either side of the pulley.

Eitan Levy
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Homework Statement


A boy holds the rope so it can't move. What is the tension in point A?

Homework Equations


ma=F

The Attempt at a Solution


The answer in the book is 4N. I understand how to get it, but, I believe that the tension of S1 would be 9N (due to m1), and then the forces on m2 would be gravity, TA and TS1. Together, we don't get zero, but 3N. Is there a problem or am I wrong?
 

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Eitan Levy said:
A boy holds the rope so it can't move. What is the tension in point A?
No way of knowing without being told where the boy is holding the rope.
If somewhere on S1 then the tension at A will indeed be 4N (and your other method doesn't work because you are not taking into account the force the boy exerts).
If somewhere on S2 then it will depend exactly where on S2.
 
haruspex said:
No way of knowing without being told where the boy is holding the rope.
If somewhere on S1 then the tension at A will indeed be 4N (and your other method doesn't work because you are not taking into account the force the boy exerts).
If somewhere on S2 then it will depend exactly where on S2.
He holds S1, but I still don't understand how the forces will work.
 
Eitan Levy said:
He holds S1, but I still don't understand how the forces will work.
So add that force, and note that the tensions will be different each side of where that force is applied.
 
haruspex said:
So add that force, and note that the tensions will be different each side of where that force is applied.
So on the left side it would be 9N and on the right side 6N?
 
Eitan Levy said:
So on the left side it would be 9N and on the right side 6N?
If the boy holds S1 at the top, yes. But he might be holding it half way up one side.
 

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