How to draw pulley system diagram to find the relationship between two objects?

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Homework Help Overview

The discussion revolves around a pulley system involving two blocks, A and B, where the original poster seeks to establish the relationship between their accelerations. The context includes a diagram that is intended to illustrate the system but has been identified as incorrect by the original poster.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants explore the relationship between the accelerations of the two blocks, questioning the signs and directions of their movements. There is a focus on how to correctly represent the system in a diagram and the implications of different assumptions about direction.

Discussion Status

The discussion is ongoing, with participants providing insights into the relationship between the movements of the blocks and the implications for the diagram. Some guidance has been offered regarding the direction of forces and accelerations, which seems to be aiding the original poster's understanding.

Contextual Notes

There are mentions of confusion regarding the signs of the accelerations due to the differing directions of movement (up/down for block A and left/right for block B). The original poster has also indicated that their initial assumptions about the diagram may have been incorrect.

theBEAST
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Homework Statement


Here is the diagram and I need to find a relationship between the acceleration in block A and the acceleration in block B.
http://dl.dropbox.com/u/64325990/HW%20Pictures/datum2.PNG

The Attempt at a Solution


Here is my diagram, which is wrong:
http://dl.dropbox.com/u/64325990/HW%20Pictures/Photo%202012-04-08%209%2039%2025%20PM.jpg

It is wrong in that there shouldn't be a negative sign in front of aB. Instead 3aA=2aB is the correct answer. However I do not know how to derive that answer and where to put my datum. I need help with the diagram, thanks!
 
Last edited by a moderator:
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If A moves downward B will move to the left and SB will decrease. You supposed aB in the direction of growing sA.ehild
 
Last edited:
ehild said:
If A moves downward B will move to the left and SB will decrease. You supposed aB in the direction of growing sA.ehild

I am still confused... So do you mean that with what I did, I can still get the correct answer? This is really confusing because one object is moving up/down while the other is left/right so I don't know how to set up the signs and how to define the directions.

I forgot to mention in the question that block A accelerates downwards and block B is to the left according to the question. I did the problem with 3a_A=-2a_B and I go the wrong answer. When I used 3a_A=2a_B I got the right answer.
 
Last edited:
OH I just had an epiphany, so are you saying that the way I drew my diagram would be wrong in that s_B is pointing to the right when it should be pointing to the left since the question tells me a_B is pointing to the left?
 
It depends what you mean on S. If it is the length of a piece of thread, 3SA+2SB =L is right. If it is displacement of a block in the "natural direction of moving) 3SA=2SB.

When the motion goes on along a thread, it can be considered one-dimensional. The actual direction (horizontal, vertical, along a slope... ) is irrelevant. Fit the direction of forces to the supposed direction of the accelerations.

ehild
 
ehild said:
Fit the direction of forces to the supposed direction of the accelerations.

ehild

This is what I am doing now and it has been working great so far! Thanks for your help!
 
You are welcome. :smile:

ehild
 

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