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Block and Pulley Velocities

  1. Feb 12, 2014 #1
    1. The problem statement, all variables and given/known data
    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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    Last edited: Feb 12, 2014
  2. jcsd
  3. Feb 12, 2014 #2

    BvU

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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,
     
  4. Feb 12, 2014 #3

    BvU

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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....
     
  5. Feb 12, 2014 #4
    Plz explain what do you want to say?
    Is answer 2VP correct?
     
  6. Feb 12, 2014 #5

    BvU

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    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!
     
  7. Feb 12, 2014 #6
    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
     
  8. Feb 12, 2014 #7

    haruspex

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    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.)
     
  9. Feb 12, 2014 #8
    So for const direction of pull, is The answer is VP(2+cosβ) ?
    If not, Plz tell me how to solve
    PLZ
     
    Last edited: Feb 12, 2014
  10. Feb 12, 2014 #9

    haruspex

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    No, the given answer is correct for that case. You didn't answer my first two questions:
     
  11. Feb 12, 2014 #10
    1.Decreases by X
    2.cant figure out
     
  12. Feb 12, 2014 #11

    haruspex

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    Does the string as a whole change length?
     
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