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Homework Help: Physics Problem

  1. Jul 7, 2005 #1
    https://www.physicsforums.com/attachment.php?attachmentid=4224&stc=1

    The angle between A and horizontal is 60, angle between D and horizontal is 70,angle between BC and horizontal is 30. Mass of P is 2kg.
    Find: (a) Tension in AB, BC and CD
    (b) Mass of Q.

    I have been trying to solve this question by using the following 3 equations but I still cant get the answer: ABsin60 + BCsin30 = 2g
    CDsin70 + BCsin30 = m(Q)g
    ABcos60 = CDcos70 + BCcos30
    Can anyone help me to solve this question? Thanks. :smile:
     

    Attached Files:

  2. jcsd
  3. Jul 7, 2005 #2

    Doc Al

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    Staff: Mentor

    This one's OK (vertical forces at B).
    Vertical forces at C. Problem: Tension "BC" pulls down at C.
    Where did this one come from? Instead, consider horizontal forces at B and C.
     
  4. Jul 7, 2005 #3
    The part I dont understand the most is about the horizontal force....
    Is ABcos60 = BC cos30 and BCcos30 = CDcos70 ??
     
  5. Jul 7, 2005 #4

    Doc Al

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    Staff: Mentor

    That looks correct to me.
     
  6. Jul 7, 2005 #5
    Some more...may I know how to identify the direction of tension BC vertically?
    Thanks.
     
  7. Jul 7, 2005 #6

    Doc Al

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    Staff: Mentor

    I'm not sure I understand your question. The angle of BC is given. The tension pulls upwards at B and downwards at C.
     
  8. Jul 7, 2005 #7
    Thanks a lot...
     
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