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Homework Help: Please help find my error with this velocity diagram?

  1. Nov 12, 2013 #1
    1. The problem statement, all variables and given/known data

    Find piston velocity and
    Find angular velocity AB about A.


    I apologise if the image is poor! If very bad I'll elaborate in writing the values....

    2. Relevant equations

    Draw a velocity vector diagram to solve the above.
    Also use 2∏RPM / 60 to convert crank speed from 3000 rpm to 314.16 rad / s
    And then Wr to get 31.416 metres per sec angular speed of point A
    And for "angular velocity AB about A" used Wab = Vab / Length ab = 17.4 / 0.2 = 87 rad / s. (But book says answer should be 21.25??

    3. The attempt at a solution

    Attached below my 'sketch'. I did this to clarify as well:

    This is a simple slider crank example from one of my own books; the book gives the answers but not the method.

    The answers given by book are:

    34 m/s piston velocity (and I get this right)

    21.25rad/s for angular velocity of AB about A. (I get 87 rad/s????)

    Please help, question and my space / velocity diagram below.

    Basically I did this:

    1/ Drew question schematic to scale
    2/ Drew Vector Vao (90 degrees to AO)
    3/ Drew Vector Vab (90 degrees to AB at piston)
    4/ Convert crank rpm to rad/s and then calculate Voa ( Voa = crank rad/s x length 0.1m)
    5/ Draw vectors using the angles calculated from above methods (Voa being 30 degrees on right hand side from horizontal)
    (Vab being 65 degrees from left hand side horizontal.
    6/ Once first vector drawn point A is know, so join next (Vab) to a, and finally measure ao (Vao) to find o, and then drew horizontally across to find b.
    7/ Then do Wab = Vab / length ab = 17.4/0.2 = 87 rad/s

    Answer vector.jpg
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 12, 2013 #2
    It's OK guys - tutor took a look and I am right, the book is wrong!
    Thanks though..
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