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Find the Force this man needs

  1. Nov 7, 2009 #1
    I am unsure of how to go about this problem. So if anyone could help me by going step by step through it, showing all equations and numbers used, and then showing me the answers so I can make sure I get the same thing, I would appreciate it.


    A man is attempting to raise a 7.5 m-long, 28 kg flagpole that has a hinge at the base by pulling on a rope attached to the top of the pole, as shown in the figure below. With what force does the man have to pull on the rope to hold the pole motionless in this position?

    http://session.masteringphysics.com/problemAsset/1013609/7/jfk.Figure.P08.39.jpg

    **(The picture I provided the link for will need to be used to answer the problem.)**
     
  2. jcsd
  3. Nov 7, 2009 #2

    rl.bhat

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    Homework Helper

    So if anyone could help me by going step by step through it, showing all equations and numbers used, and then showing me the answers
    If we have to do all these things, then what is your task? Our job is to provide the hints, so that you can understand the problem. Try to answer the following questions.
    1) When the pole will be motionless in the given position?
    2) what are the forces acting on the pole?
    Resolve these forces into two components. i) perpendicular to the pole and ii) along the pole. The perpendicular components produce the torques on the pole.
     
  4. Nov 7, 2009 #3
    Nobody has to do anything. I am just trying to get some extra help because I don't understand Physics very well. I need that extra boost to get me where I need to be. I don't want to be given the answers because then I learn and gain nothing. I just want to make sure people tell me step by step what I need to know or do. I don't want to be lost when they tell me something.

    Now, as for your questions: You're first one, or #1, didn't make sense.
    #2: There is a weight from the center of the pole going downward, and there is a normal force from the surface onto the pole, pointing upward. That is the only forces I can see. The tension is in the rope, so that isn't included in your question.
     
  5. Nov 7, 2009 #4
    What you need to do is to find the moments- clockwise and anticlockwise at the hinge.
    CW moment Weight of the pole acting at the Centre of gravity=mg. Multiply this by the effective distance from the hinge ie 7.5/2 x cos 30.
    Similarly find the other moments. Equate the CW moments to the ACW moments and you should get the answer.
     
  6. Nov 8, 2009 #5
    Well, when I did the first thing you said, I got 772.3058. Is that a weight, or a force? And how does that help with the rest of my problem?
     
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