Understanding Velocity in a Massless, Frictionless Pulley System

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Homework Help Overview

The discussion revolves around a problem involving a massless, frictionless pulley system where participants are tasked with finding the velocity V in terms of another velocity VP, while ignoring gravity and other forces. The original poster presents an initial solution and expresses confusion regarding a discrepancy with the provided answer key.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore the relationship between the movement of the pulley and the resulting changes in string length. Some question the assumptions made about the system, particularly regarding the angle of the string and the direction of pull. Others suggest clarifying the problem statement and listing known variables and equations to facilitate understanding.

Discussion Status

The discussion is ongoing, with various interpretations being explored. Some participants have provided guidance on how to structure the problem for clarity, while others are seeking to understand the implications of different assumptions. There is no explicit consensus on the correct approach or answer at this stage.

Contextual Notes

Participants note that the problem is not fully specified, leading to multiple interpretations. There are discussions about what variables should remain constant and how they affect the outcome. Additionally, there is a mention of forum rules that may limit the expression of gratitude, indicating a collaborative environment.

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Homework Statement


Find V if the pulley is massless, friction less and
Can take any no of variables but V is be in terms of VP.Ignore gravity and other forces
attachment.php?attachmentid=66537&stc=1&d=1392225169.jpg


2. The attempt at a solution
If Pulley comes down by X, the length of string towards left(the side not fixed) will increase by 2X
Therefore, If in same time pulley comes down by VP, String will increase at rate of V=2VP.
.'. the answer is V=2VP
But, The answer in key states V = VP(1+cosβ)
Where am I going wrong?
 

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If the pulley is hanging in the air, everything will come down very quickly. If it is attached to something fixed, and the lower right represents Earth -- which is generally considered fixed as well -- Any amount of V (what is it, actually?) won't have an effect, no matter what.

Please rewrite 1, 2 and 3. We will try to help, but this requires telepathy or clearsightedness,
 
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I see you already have an answer. Now the questions. i.e. the question the problem asks and the question you are asking...
 
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BvU said:
I see you already have an answer. Now the questions. i.e. the question the problem asks and the question you are asking...
Plz explain what do you want to say?
Is answer 2VP correct?
 
What I want to say is: 1. State the problem, list known variables and data. 2. List the relevant equations 3. Show what you've done so far and where you get stuck.

Do all three in such a way that someone else has a good chance to understand.

This soounds rude, but the idea is that the template is really useful to help both the one who is posting and potential helpers.

At the moment I can't help you because I can't understand the problem you want to solve, nor what is withholding you from solving it. I really want to help, make it possible!
 
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BvU said:
What I want to say is: 1. State the problem, list known variables and data. 2. List the relevant equations 3. Show what you've done so far and where you get stuck.

Do all three in such a way that someone else has a good chance to understand.

This soounds rude, but the idea is that the template is really useful to help both the one who is posting and potential helpers.

At the moment I can't help you because I can't understand the problem you want to solve, nor what is withholding you from solving it. I really want to help, make it possible!
Modified The quesion.
I hope it will help u to understand my question.
I want to give u 100 thanks at once but forum rules restricts me to only one
 
The problem is not fully specified. I can justify two answers depending on what is supposed to be constant: the angle of the string or the direction of pull. To get the given answer, it has to be the direction of pull.
rajumahtora said:
If Pulley comes down by X, the length of string towards left(the side not fixed) will increase by 2X
If the pulley comes down by X, by how much is the length of the string between pulley and ground anchor reduced? So the length of the string to the left of the pulley has increased by ...?
That argument tells you how the free end moves relative to the pulley, but to get V you need to add in a component of the velocity of the pulley.
(If taking the constant angle of string interpretation, you would add the velocities as vectors instead.)
 
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haruspex said:
The problem is not fully specified. I can justify two answers depending on what is supposed to be constant: the angle of the string or the direction of pull. To get the given answer, it has to be the direction of pull.

If the pulley comes down by X, by how much is the length of the string between pulley and ground anchor reduced? So the length of the string to the left of the pulley has increased by ...?
That argument tells you how the free end moves relative to the pulley, but to get V you need to add in a component of the velocity of the pulley.
(If taking the constant angle of string interpretation, you would add the velocities as vectors instead.)
So for const direction of pull, is The answer is VP(2+cosβ) ?
If not, Plz tell me how to solve
PLZ
 
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rajumahtora said:
So for const direction of pull, is The answer is VP(2+cosβ) ?
No, the given answer is correct for that case. You didn't answer my first two questions:
If the pulley comes down by X, by how much is the length of the string between pulley and ground anchor reduced? So the length of the string to the left of the pulley has increased by ...?
 
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haruspex said:
No, the given answer is correct for that case. You didn't answer my first two questions:
1.Decreases by X
2.cant figure out
 
  • #11
rajumahtora said:
1.Decreases by X
2.cant figure out
Does the string as a whole change length?
 

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