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

AI Thread Summary
To find the relationship between the accelerations of two blocks in a pulley system, it's essential to correctly define the directions of motion. The correct relationship is 3aA = 2aB, indicating that as block A accelerates downward, block B accelerates to the left. It's crucial to align the signs of the accelerations with the defined directions of movement to avoid confusion. Understanding the motion as one-dimensional along the thread simplifies the analysis, as the actual orientation of the motion does not affect the relationship. Properly drawing the diagram and assigning directions to the displacements will lead to accurate results.
theBEAST
Messages
361
Reaction score
0

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:
Physics news on Phys.org
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
 

Similar threads

Ampere's Law and Infinite Current Sheets
Replies
3
Views
4K
Finding the acceleration of two masses on a pulley system
2
Replies
66
Views
7K
Two Blocks and a Pulley Friction Problem
Replies
2
Views
4K
Pulley system w/hanging mass, two unknown masses and Mk
Replies
7
Views
3K
How to Calculate the Mass of the Left Block in a Pulley System?
Replies
3
Views
18K
Finding the relationship between accelerations in a pulley system
Replies
2
Views
10K
System of two inclines planes and pulleys
Replies
4
Views
3K
How Do You Calculate Forces and Acceleration in a Two-Block Pulley System?
Replies
3
Views
2K
Acceleration of two masses on pulley.
Replies
1
Views
7K
How Do You Prove Static Equilibrium in a Three-Mass, Two-Pulley System?
Replies
27
Views
6K

Hot Threads

Recent Insights

Back
Top